The effect of Primobolan (methenolone enanthate; 10 mg kg −1 thrice-weekly) on total body nitrogen, potassium, phosphorus, and body-weight, muscle protein synthesis and food conversion efficiency was measured in Wistar rats during the period from weanling to maturity (22 to 64 days of age). Treated male rats exhibited a lower rate of weight gain, increased kidney weight and decreased testicular weight when compared with controls. No differences in total nitrogen, potassium or phosphorus were observed. Treated female rats had significantly increased body-weight, total body nitrogen, phosphorus, and potassium compared with female controls although, when expressed as a percentage of body weight, phosphorus content was higher in controls. Treated female rats had increased food conversion efficiency expressed relative to total body weight gain or total body nitrogen gain. Rates of synthesis of mixed skeletal muscle protein, measured at the conclusion of the study, indicated that treated females had a significantly increased rate of synthesis when compared to female controls. There was no significant difference between treated male rats and control animals.