1. Young rats were offered to appetite a semi-synthetic diet either alone or containing the beta 2-selective agonist clenbuterol (4-amino-alpha[t-butylamino)methyl]-3,5-dichlorobenzyl alcohol). 2. In female rats (starting weight 116g) the presence of the drug at daily doses greater than 10 micrograms/kg body-weight per d increased the growth of skeletal and cardiac muscle but had no stimulatory effect on the growth of the liver, gastrointestinal tract and kidney. 3. Male rats (starting weight 53 g) received clenbuterol at a daily oral dose of 200 micrograms/kg body-weight per d. Animals were slaughtered after 0, 4, 8, 11, 18, 21 and 25 d of treatment. At 4, 11, 21 and 25 d muscle protein synthesis was measured by the method of Garlick et al. (1980). Although clenbuterol increased the rate of protein and RNA accretion in gastrocnemius and soleus muscles, protein synthesis was not increased. 4. The results suggested that the drug had a rapid, perhaps direct, inhibitory effect on protein degradation. It is concluded that the growth-promoting effect of clenbuterol may be specific to muscle and that the drug may act in a novel manner which circumvents the physiological mechanisms responsible for the control of muscle growth.