Description: OBJECTIVE To determine the anabolic and lipolytic effects of a low dosage of clenbuterol administered orally in working and nonworking equids. ANIMALS 8 nonworking horses and 47 polo ponies in active training. PROCEDURES Each polo pony continued training and received either clenbuterol (0.8 gg/kg) or an equal volume of corn syrup (placebo) orally twice daily for 21 days, and then was evaluated for another 21-day period. Nonworking horses received clenbuterol or placebo at the same dosage for 21 days in a crossover trial (2 treatments/horse). For working and nonworking horses, percentage body fat (PBF) was estimated before treatment and then 2 and 3 times/wk, respectively. Body weight was measured at intervals. RESULTS Full data sets were not available for 8 working horses. For working horses, a significant treatment effect of clenbuterol was detected by day 3 and continued through the last day of treatment; at day 21, the mean change in PBF from baseline following clenbuterol or placebo treatment was -0.80 PROCENT (representing a 12 PROCENT decrease in PBF) and -0.32 PROCENT, respectively. By day 32 through 42 (without treatment), PBF change did not differ between groups. When treated with clenbuterol, the nonworking horses had a similar mean change in PBF from baseline from day 6 onward, which peaked at -0.75 PROCENT on day 18 (an 8 PROCENT decrease in PBF).Time and treatment had no significant effect on body weight in either experiment. CONCLUSIONS AND CLINICAL RELEVANCE Among the study equids, long-term low-dose clenbuterol administration resulted in significant decreases in body fat with no loss in body weight