The effects of the anabolic steroid stanozolol on whole body and muscle growth and protein metabolism in the rat have been examined. No responses could be found in normal well-fed male rats. Female rats responded to 1 mg/kg per day with an increased body and skeletal muscle growth rate and an increase in muscle protein synthesis. The anabolic action on muscle protein synthesis was due to increased RNA concentration with no change in the rate of protein synthesis per unit RNA (KRNA). Investigation of any anticatabolic effects of stanozolol treatment in male rats deprived of food for 24 h indicated no response of protein balance and turnover. However, rats treated with catabolic doses of corticosterone (50 mg/kg per day) did respond to stanozolol with decreased muscle growth inhibition due to better-maintained muscle protein synthesis. The latter response was due to a reversal of the corticosterone-induced reduction of KRNA, but with no effect on RNA concentration. Thus there appear to be at least two effects of stanozolol; an anabolic action evident only in female rats, involving increased muscle RNA concentrations, and an anticatabolic action involving inhibition of the corticosterone-induced fall in muscle RNA activity. In both cases, stanozolol influenced muscle protein synthesis with no evident effects on protein degradation.