Personal trainers record audio or video of their workout instructions and make them available through a Web site. Exercisers pay a fee to download the MP3s or videos to their iPods or other MP3 player, or burn them to a CD.

Fees range from a few dollars a workout to a monthly subscription fee. Downloads include treadmill, elliptical or stationary bike workouts, meditation, yoga, and Pilates.

For much less than it would take to hire a personal trainer, exercisers can work out under the instruction of experts who help celebrities get their enviable bods.

With audio alone, it's sometimes hard to figure out exact poses and forms, especially for yoga and Pilates.

To help avoid injury, Zeena Dhalla, owner of The Athletic Club for Women in Newport Beach, Calif., suggests shelling out for a personal trainer at least once and have him or her do the downloaded workout with you.

The trainer can help explain poses and make suggestions before you head out on your own.

"They're a great idea, great concept, but it doesn't preclude someone from seeing a professional to get some help," she said.

Podfitness.com

Cost: A $19.95 subscription will get you two downloads a day.

Overview: Here you'll find a celebrity trainer bonanza. In each trainer's profile, you'll find their qualifications and an impressive list of celebrity bodies they've whipped into shape.

Trainers include notables such as Kathy Smith and Kathy Kaehler and, believe it or not, Danny Bonaduce. Workouts include weight training, Pilates, yoga, elliptical, stretching, pre-natal, walking, running and yoga.

Plus: You can mix the audio instructions with your own playlist, so you hear music you like.

Minuses: Downloading a workout seemed easy enough: fill out a profile detailing your fitness level and experience, choose a trainer, select a workout. Then the site will mix the workout audio instruction with the playlist you've created.

Switching between workouts and trainers was clunky, error messages were plentiful, and pages were slow to load and refresh, even with our office's lightning-quick connection.

Still, when I heard the sounds of Gwen Stefani and the Foo Fighters pumping beneath Jeanette Jenkin's instructions (trainer of celebrities like Christina Applegate and Carmen Electra), I was pleasantly surprised.

One major glitch: A third download (a yoga workout with Cyndi Lee) mixed the workout with the audio track from Jenkin's treadmill workout. To its credit, customer service was easy to contact and after you get used to the site's quirks, the downloading really is easy.

At press time, this site was not compatible with Mac OS X.

Cost: Individual downloads start at $3.99. Memberships are $9.99 a month or $79.99 for the year.

Overview: Co-founded by fitness expert Grace Lazenby, the audio workout programs include treadmill, cycle, stair climber, strength, row, stretch, yoga and Pilates.

Plus: Workouts come with printable PDFs that illustrate poses. Short audio samples of workouts are available, and downloading my chosen workouts was a breeze.

Minuses: The music was so bad, it was distracting. Songs reminded me of early boy bands, "The Sims" and bad electro-pop.

PumpOne.com

Cost: Available in single workouts and sets, $10 to $70

Overview: Not sure what to do with that barbell at the gym? Download a series of photos detailing exercises to an MP3 player with a color screen, PDA, or computer. Workouts include strength, golf conditioning, women's and men's specific programs, Pilates, yoga and abs.

Cost: $1.50 to $5 for 15-20 minute workouts.

Overview: Download audio instructions for meditation and a variety of yoga styles including Anusara, flow, and pre-natal. "Coffee break yoga" for work, and a suite designed to help relieve travel stress are also available. Samples are available for some downloads.

Cost: $14.95 to $20

Overview: Audio workouts for cardio machines. Audio samples available. A quick-start guide helps you assess your fitness levels, offers an overview of some pieces of cardio equipment, heart rate zone, and the structure of a typical Cardio Coach workout.

Muscleaudio.com

Cost: Packages start at $29.95.

Overview: David Greenwalt is ready to pump you up. This Web site is made just for building muscle with its audio weight training programs.

One major downside: You have to enter an e-mail address to get into the site. Then, like it or not, you'll start receiving the Muscle Audio tips e-mail newsletter.

Cost: Prices start at $4.95.

Overview: Audio and printable guides for yoga, Pilates, strength training, running, power walking, weight loss, and golf. Also a wide array of non-fitness topics like business, knitting, massage, fear of flying, personal growth, and travel.

Celebrity instructors include trainer Kathy Smith, and yoga instructors Seane Corn and Sara Ivanhoe.

propelfitnesswater.com/gunnar-one-on-one/

Cost: Free.

Overview: Partnered with Propel fitness water, celebrity trainer Gunnar Peterson presents mix-and match downloadable videos.

Cost: Free, but you'll have to listen to a commercial between the audio and the actual interview.

Overview: This site isn't about workout instructions, but it provides great motivation for runners, cyclists, triathletes and adventure racers with audio interviews of athletes, fitness experts, and book excerpts.