When Jane Fonda crows "This is it!" at the beginning of her "New Workout" videocassette, you can almost hear the cheers of countless aerobic addicts who have awaited her latest electronic exercise venture with the keen anticipation of TV junkies hungering for the next installment of "Miami Vice."

In the first month Jane Fonda's "New Workout" was available in video stores, more than 100,000 were sold. It is already No. 3 on the Billboard magazine charts -- giving Fonda three videos in the top 10. The original "Workout" is now No. 4 and has been at or near the top of the chart for 184 weeks. Fonda's "Prime Time Workout," for older and beginning exercisers, is No. 9. Her "Workout Challenge" is popular with hard-core aerobic addicts, and her "Workout for Pregnancy, Birth and Recovery" is a must for modern maternity.

Fonda has won these loyal fitness fans for good reason. Unlike other celebrity workouts -- such as Raquel Welch's impressive but impractical "Total Beauty" yoga videocassette, or Bubba Smith's worthlessly laughable "Bubba Until It Hurts" -- Fonda's programs are fun, helpful and inspiring. Next to Kathy Smith's "Ultimate Video Workout," Fonda's exercise videocassettes are the best on the market.

Like the original, "New Workout" features both a beginner and an advanced class. But unlike the original, Fonda now conveys an attitude of encouragment, support and camaraderie. Critics of Fonda's original "Workout" chided her for coming on like a brutal drill sergeant, commanding exercisers to "go for the burn." Also, movements were criticized as being too jerky, with the potential for injury from popping rather than stretching muscles.

Fonda's attitude has lightened up in each subsequent video workout, including "New Workout." After countless repetitions of a difficult exercise, she grins conspiratorially and urgers exercisers on with, "Hang in there, we're almost done!" This is in sharp contrast to her original "Workout," where -- in a similar situation -- she tosses her perfectly coiffed head and barks, "C'mon. If I can do it, you can do it."

The "New Workout's" best feature is the expanded aerobics section. There is a 12-minute aerobics segment in the 35-minute beginner class and a 16-minute aerobics segment in the 55-minute advanced class. In consultation with Dr. James Garrick, director of the sports and dance medicine clinic at St. Francis Memorial Hospital in San Francisco, Fonda has cleverly combined the arm workout with the aerobics section so exercisers can work the arms and the cardiovascular system at the same time.

She also has an excellent abdominal workout in the advanced section, and her tried and true leg and thigh workout are in both the beginners and advanced class.

It's too bad she didn't expand the aerobics segment to 20 minutes in the advanced class, however, since getting the heart rate up for 20 minutes three times a week is the generally accepted minimum standard for cardiovascular fitness. It's also a shame she doesn't get better music. The sing-along song that accompanies the aerobic workout -- "There's So Much More to You Than Meets the Eye" -- is silly, and it's not enhanced by being lip-synched by a class member.

It is fun, however, to see some familiar faces in the class. Peggy Lipton, the mini-skirted member of the '60s TV show "The Mod Squad," sweats along with Jane, as does Marianne Black, who was last seen working out while nine months pregnant in Fonda's "Workout for Pregnancy Birth and Recovery."

Fonda says her goal is to create a full library of exercise tapes, so that "depending on the mood one is in, you can choose from a variety of workouts." The first five are impressive -- here's to Volume 6.