Exercise that makes you sweat builds energy" Any exercise expert worth her wall-bars - Barbara Pealman, Lydia Bach, etc - stresses that extra energy can come from exercise, that the more you do, the more energy you can build. At forty-three, Jane Fonda has never looked better and couldn't agree more, attributing her new boost of amazing stamina to a life of exercise.

Two years ago, she decided to open her first workout studio, with classes going on from 7:30 A.M. to 9:00 P.M. - "because I knew many people had to fit classes either before or after work." Her first book, "Jane Fonda's Workout Book" (Simon and Schuster), out in November, will document The Workout's classes and give step-by-step instruction on how to follow the programs at home. Again, because of her personal experience, "I couldn't exercise when I was pregnant; I didn't know what I could safely do." Jane was one of the first to introduce exercises specifically designed for pregnant women, the "P.B.R" (for "Pregnancy, Birth and Recovery") exercises, to help women stay fit during pregnancy, keep muscles in top condition, for the birth and swiftly bring about "shape recovery" afterwards. When the workout arrives on the east coast within the year, with a large pool on the premises, Jane intends to introduce water workouts for "PBR" clients, too. Where can we expect to find the next Workout opening? "Where I can duplicate the quality of the staff." Says Jane. "They're the best - and that's the way it has to be"