Periodic replacement of an automatic transmission's hydraulic fluid and internal filter is an important but often neglected routine service. Part of the reason for this is that the average Do it Yourselfer doesn't think about doing it, or thinks it's too hard. The other is that the automakers seem in many cases to de-emphasize regular transmission service - sometimes advertising "lifetime" fills or very long intervals between fluid/filter changes - as a way to lure buyers with what sounds like low maintenance costs. But don't buy it. Regular transmission fluid/filter changes are as important to the longevity and good performance of an automatic transmission as regular oil and filter changes are to the long-term health of your engine. And don't be afraid to do it yourself, either.

Here's how:

* Determine the appropriate fluid/filter changeout intervals. This information should be in your vehicle owner's manual under "service." However, be wary of any changeout interval that is greater than 3-4 years and 30,000 miles or so. While it might be ok to run the transmission for many more miles without changing the fluid or filter, it's a good bet that by 30,000 miles and 3-4 years the fluid has begun to deteriorate and the filter is at least partially filled with the normal debris of wear and tear. Replacing the fluid and filter every 30,000 miles or so may be more frequent than some manufacturers recommend, but it is probably the smart move if you want to get the maximum life/economy out of your vehicle - and in any case will not cause any harm.

* Determine the amount of fresh fluid you will need after draining the transmission pan. The info should be under "capacities" in your owner's manual. If it's not, you may need to read/buy a service manual for your vehicle; these are available at most auto parts stores (such as NAPA, AutoZone, etc.). 4-5 quarts is typical.

* Be sure to get exactly the type of fluid recommended by the manufacturer of your vehicle. Some makes (imports especially) are very sensitive and use of other than the recommended type of fluid may lead to problems you don't want. Same with the filter. Use only the type explicitly recommended by the manufacturer of your vehicle. Filters may come as a "kit" that includes a new gasket for the transmission pan. If you don't get the gasket with the filter, be sure to buy one separately. (It's sometimes possible to re-use the old gasket but in general it's good policy to always replace it with a fresh one to reduce the chances of drips/leaks, etc.)

* Get a little container of petroleum jelly (Vaseline), plus a razor blade/scraper and a large capacity drain pan - the wider the better (to catch as much fluid as possible). Have lots of rags/paper towels on hand. This can be a messy job!

* Let the car cool for at least an hour before working on it. Automatic transmission fluid can get very hot and most modern cars do not have a drain plug in the pan, which means you are likely to get splashed with fluid.

* Raise/support the vehicle sufficiently so that you can crawl underneath and get at the transmission oil pan. The transmission pan should look similar to the engine oil pan; it is typically held in place by 12 or so small bolts. You will need a basic socket set/ratchet to remove these bolts. In all likelihood (unless the car is older than about model year 1981) these bolts will be metric.

* If the pan is roughly shaped like a square, remove all the bolts except one bolt at each corner. Keep track of the bolts by dropping them into an old coffee can or something like that as you work. (You may also need to remove a bracket that holds the shift linkage cable.)

* Place the catch pan directly underneath the rearmost part of the pan and gently begin to loosen the back two bolts; fluid will begin to drain out of the pan. Ease the bolts out some more to drain more of the fluid. Keep doing this until the pan is partially off and enough of the fluid has drained off such that you can comfortably support/move the pan up and down with your hand. Now remove the remaining bolts, supporting the pan as you go, and gently ease it into your catch pan. Let the transmission continue to drain/drip until as much fluid is drained as is going to drain.

* Locate the filter. It should be immediately visible once the pan is off and is typically held in place by a couple of smaller bolts/screws. (Filter designs differ from brand of car to brand of car; more detailed info about your specific vehicle is readily available in the repair manuals mentioned earlier.)

* Carefully remove the filter - being sure to also remove any gaskets/o-rings that should come out with the filter.

* Inspect the old filter and the transmission pan. A small quantity of silty material and very fine metallic particles is usually considered normal wear and tear. (If you discover larger pieces that look like bits of shredded tinfoil, you may be looking at signs of serious transmission problems.)

* Using a lint-free rag, clean out the transmission pan and remove all traces of the old gasket using a scraper or razor blade, if necessary.

* Be sure the surface of the transmission case where the pan installs is also free of any old gasket material.

* Install the new filter; be sure it is installed correctly. Double check your work as once the pan is back on you will not be able to immediately see whether there's a problem (as you would if you didn't install the engine oil filter properly).

* Reinstall the pan. Use the Vaseline to keep the gasket in place/lined up with the bolt holes during this process. Start all bolts by hand to avoid cross-threading any of them. Once all bolts are threaded, gently tighten them. Start at the middle and work your way outward, tightening the bolts gradually and sequentially. Be very careful not to overtighten these bolts. Most transmission cases are made of relatively soft aluminum. Ideally, use a torque wrench and tighten the bolts to the exact specifications mentioned in the repair manual. But if you don't have a manual or torque wrench, slightly more than "hand tight" should be about right. (It is better to err on the side of caution here. If the bolts are still a little loose and the pan seeps or drips a little, you can always tighten the bolts some more. If, on the other hand, you overtighten the bolts you risk ruining the threads, warping the pan or - worst of all - cracking the transmission case.)

* Reinstall any brackets, cables, etc. Lower the car (it should be level or as close to level as possible to get an accurate fill reading on the transmission dipstick).

* Using a funnel, carefully pour in the new fluid. To be safe, pour about a pint less than the manual specifies. This is done to make sure you don't overfill the transmission. Check the level on the dipstick. It should be just under the "full" line.

* Start the engine and with your foot on the brake, put the transmission in Drive, then Neutral, then Reverse (as well as any other ranges). Let it idle for a couple of minutes. Be sure there is no leaking underneath the car.

* Recheck the level and carefully top off until the level is at the "full" mark. Drive the car for 5-10 minutes, park on a level surface and recheck the fluid level again, adding more fluid if necessary.

The transmission should shift normally; if you notice any change in operating characteristics, do not continue to drive the vehicle. Re-check the fluid level first but if that checks out you may have done something wrong such as improperly installed the filter. It may be necessary to raise the vehicle, drop the pan and re-check your work. But whatever you do, do not continue to drive the vehicle. An automatic transmission can be destroyed by just a few miles of operation with some basically minor problem.

I mention this just as a caution. It's not likely to happen. But it's very important to not drive the vehicle if the transmission starts acting up. If something isn't right, you should either check it yourself or get the car checked by an expert.