Clenbuterol is a relatively selective β2-adrenergic partial agonist that has bronchodilator activity. This drug has been investigated as a potential countermeasure to microgravity- or disuse-induced skeletal muscle atrophy because of presumed anabolic effects. The purpose of this study was to: 1) analyze the anabolic effect of clenbuterol's (-)-R and (+)-S enantiomers (0.2 mg/kg) on muscles (cardiac and skeletal) and other organs; and 2) compare responses of enantiomers to the racemate (0.4 mg/kg and 1.0 mg/kg). Male Sprague Dawley rats were treated with: a) racemic clenbuterol (rac-clenbuterol, 0.4 or 1.0 mg/kg); b) enantiomers [clenbuterol (-)-R or (+)-S]; or c) vehicle (1.0 mL/kg buffered saline). Anabolic activity was determined by measuring tissue mass and protein content. HPLC teicoplanin chiral stationary phase was used to directly resolve racemic clenbuterol to its individual enantiomers. In skeletal muscle, both enantiomers had equal anabolic activity, and the effects were muscle- and anatomic region-specific in magnitude. Although the enantiomers did not affect the ventricular mass to body weight ratio, clenbuterol (+)-S induced a small but significant increase in ventricular mass. Both clenbuterol enantiomers produced significant increases in skeletal muscle mass, while being less active in producing cardiac ventricular muscle hypertrophy than the racemic mixture. 637-648 (C) 2000 Wiley-Liss, Inc.