You are floating weightless in a warm, still place where only now and then do feathery tendrils of sensation tickle your naked body. The darkness is so thick in here that it feels as if black blinders have been strapped to your eyes. But your eyes are wide open and nothing covers them but darkness. Water fills your ears and blankets the eardrums, shuts off the sound and substitutes a thick fuzzy silence, penetrated only by your muffled heartbeat. You're in an isolation tank in a modest office on Bayard Street in Pacific Beach, where you pay nine dollars an hour for this experience.

This particular isolation tank resembles an oversized coffin except that it's bright blue and has an angled-trap door cut into one end. The design is a standard one according to Oren Leblang and Vicky Schindler, who opened the tank to the public about three weeks ago. Leblang ,is a certified rolfer who uses the office for his work with the deep message technique (dubbed "rolfing" after its creator, Ida Rolf), while Schindler is getting her Ph.D. in psychology at USIU and running the tank rental operation. Years ago, she read about early experiments with isolation tank.s, work started in the 1950s by John Lilly, the controversial neurologist/dolphin researcher/philosopher. Lilly first began climbing into lightIess, soundless receptacles because he was interested in the impact of extended solitude, but he soon' branched into experiments combining sensory deprivation with drugs. Schindler says subsequent researchers refined the' tank design to its current elements and its primary use - as a tool for achieving intense relaxation.

"The basic thing about the box is to minimize as much sensory sensation as possible," the woman explains. The tank contains about nine inches of water, heated to ninety-three degrees, not hot enough to drain users of energy but sufficiently warm to minimize skin sensation. Dissolved in the water is about 750 pounds of Epsom salt, which Schindler says provides enough buoyancy to float any human body, while leaving the ears below the water line, where the water screens out all but a hundredth of the air-carried sounds. "Everything that can relax easily relaxes incredibly easily in the tank," interjects Leblang, who recommends its use to his rolfees. "The experience is very cozy. It's a lot like being in bed, except that even when you're lying in bed, your muscles are working to support you. In the tank, those muscles can completely let go."

And that's normally all that happens, Schindler has found herself explaining to. callers seeking the isolation as a source of hallucinations. She acknowledges that some tank experimenters have seen such effects, but only when they set up conditions which much more rigorously blocked out all sensation and had their subjects endure the isolation for much longer periods. In contrast, Schindler's tank exposes its users to small amounts of sound and tactile sensations and isolates them usually for only an hour or two at a time. Also, the psychologist screens out people who've just ingested heavy drugs. She says that when she was thinking about opening the tank to the public, she called another facility which has operated in Denver, Colorado, for several years and found its operators hadn't experienced a single case of a patron hallucinating. She also learned, however, that Denver policemen and air traffic controllers are working with the tank facility for stress reduction.

Schindler says one distortion which does occur involves time; customers regularly swear that less than an hour has passed. "I finally had to put out a clock. People think I'm ripping them off. I had somebody come in last night for an hour and a half and he insisted he's only taken forty breaths in that time!" She says local interest in the tank has been high, already attracting more than sixty users, many of whom approach the horizontal box nervously. To those worrying about an attack of claustrophobia, Schindler advises them to lay with their heads right under the door, which can be opened with a gentle push. "But no one's gotten out early. "

She says enthusiasm for the tanks nationally has developed to the point where one Los Angeles firm is marketing home models, now selling for about $2500; the San Diego couple consulted with that manufacturer before building their own facility. The woman acknowledges, however, that benefits from the isolation tank don't 'differ much from those experienced by serious yoga practitioners. Still, she has a ready defense for the watery alternative. "I've never been able to sit down and meditate, I find that really difficult. But the tank is easy. That's why I call it the American way." Adds her partner, "Robin Williams of Mork and Mindy has one of his own, and he calls it Caucasian meditation."