Adult white Leghorn chickens exhibited a sexual dimorphism in hepatic microsomal monooxygenases determined from the concentrations of total cytochromes P450 and b5, and the metabolism of drug (hexobarbital, coumarin and ethoxyresorufin) and steroid (androstenedione and testosterone) substrates that were 2- to 4-fold greater in roosters than in hens. Caponizing at 6 weeks of age reduced the activities of the monooxygenases to levels comparable to those found in intact hens. In spite of the fact that testosterone replacement maximally stimulated comb growth in the capons and elevated (i.e. masculinized) hepatic monooxygenase activities in the hens to male-like levels, androgen replacement was ineffective in increasing the subnormal enzyme levels in the capons. While the failure of testosterone administration to restore monooxygenase levels in the capons may be explained by the immaturity of the birds at orchiectomy, the present results demonstrate, that like some mammals, birds may display gender differences in hepatic monooxygenases that are regulated by the testes.