LAST WEEK, when Washington's air was heavy with humidity and pollution, two friends and I tossed four inner tubes in the car trunk, headed toward Luray, Va., and spent the late afternoon tubing down the South Fork of the Shenandoah.

Silently sliding down the river on our tubes, we watched wild ducks hurry toward the bank. Cardinals sang in the sycamores; dragonflies perched on our legs. When we leaned over the tubes and peeked at the river's rocky bottom, tiny bass darted for cover.

Tow hours later we pulled ourselves from the river and retrieved the car. We were tired but mellow and we marveled at the inner peace that can be found in an afternoon on inner tubes.

Tubing is not competitive. There is no prominent association of tubers that gives lessons in correct methods. There is not Tuber Magazine to rate brands of inner tubes. Barring brainstorms by network executives, there is no televised celebrity tubing.

Tubing thrives on informality. When the weather gets hot, tubers simply float down their favorite stream, relax, enjoy the scenery and eventually get out of the water. Sometimes tubers take along beer in a plastic bag with ice. The bag serves as a refrigerator and trash container. It is sealed and tied to a "mule" tube. That's as fancy as tubing gets.

Sipping cold beer while floating down a stream is like eating Kansas City barbecue. The delightful tastes, aromas and sights of the experience are stored in your subconscious to be called up on dull, rainy March afternoons.

The appeal of tubing is its lazy pace - a trip of two miles takes at least three hours. Most of a tubing trip is spent moseying, but occasional ripples appear. Then the current speeds and tubers lift their tails to shoot over six-inch waves.

Tubing does have its hazards. Tetanus shots are usually recommended if you cut your leg on a sunken branch. Poison ivy thrives along river banks. And a bruised posterior from bottom bumping bottom is common.

Some aficionados maintain that good tubers come only from the South, where the hot summers engender tubing's essential laid-back attitude. But there are other requirements. People terrified of dragonflies, water spiders and jumping fish had best stay home and watch golf matches.

Tubers are not missionaries and they don't seek converts. They like to have the streams to themselves, but if they meet fellow melancholy souls they will tell them about tubing.

The complete tuber needs hot weather, inner tubes, water, old sneakers and two vehicles.

Hot weather is no problem here but inner tubes, once as commonplace as convertibles, have been relegated to used tire stores.

A telephone survey of such stores listed in the Yellow Pages showed that prices range from $20 for a new 20-inch truck tube to $10 for a used tire the same size. Passenger car inner tubes with a 15-inch inside diameter cost from $5 for a new tube to $2 for a used one.

Passenger tire size is adequate for anyone smaller than Man Mountain Dean. Truck tires are the Cadillacs of tubing, but these tubes have a nasty metal air valve that can ruin the serenity of sitting on an inner tube.

For my journey down the Shenandoah I brought along five tubes. The river is a two-hour drive from Washington west on routes 166 and 211.

Before I left Washington I performed an important ritual: Testing the tubes, filling each with air from a service station air base. Three tubes quickly deflated. That was okay, because I still had enough good ones left. But the time to find the faulty tubes is when you are standing in the middle of a service station and not when you are trying to float down the middle of a river.

We picked the South Fork because it is easy to get to. A road leading down to the river joins 21 1/2 just before the highway crosses the river and there are places to park.

Earlier in the week I had tried to tube on the Patuxent River in Maryland, about 30 miles north of Washington but the water was too cold. After 10 minutes my skin was bright red, my chest was heaving from the cold and my fingers were getting stiff. I abandoned the Patuxent as a tubing waterway.

Usually a list of a state's good canoeing streams is helpful in finding good tubing streams. Tubers look for the stemas that the canoe guides list as mild.

One of the seldom-heralded benefits of tubing is that it provides a use for an overlooked resources, old sneakers.

Instead of spenidn their final days in the dark corner of a closet old sneakers can be brought into the sunshine. Tubers need them for protection against jagged rocks on the river bottom and, ironically, the holes in the tops of the shoes, the very defect that pushed them into retirement, make them useful in tubing. They drain the water.

Taking two vehicles on a tubing trip is a good practice, though not essential. One vehicle can be parked where tubers get in the water, another where the tubers leave the water.

However, a twosome of tubers with only one car can still float. Take along a bicycle, chain it to a tree near the spot you will get out of the water and pedal back to the car.

On the Shenandoah trip we didn't even need a bike, we simply jogged back to the car, a distance of a little over a half-mile.

After two hours meandering down the Shenandoah my friends and I decided to get out of the water. Slowly we walked through a set of ripples and headed toward the bank.

Ten feet from the bank, I jumped when I saw what I thought was a stick pop its head out of the water. It was a snake.

We both bolted, but in opposite directions. That is about as fast-paced as tubing should ever get.