Sex differences in hepatic microsomal cytochrome P-450 and monooxygenase activities were investigated in the marine teleosts scup (Stenotomus chrysops) and winter flounder (Pseudopleuronectes americanus). Sex differences in ethoxyresorufin O-deethylase activity reflected suppressed levels of the responsible isozyme, P-450E, in females. Microsomal estradiol (E(2)) 2-hydroxylase activity, demonstrated here for the first time in teleosts, was sexually differentiated in scup, and P-450-mediated although not by P-450E. E(2)2-hydroxylase, testosterone 6(beta)-hydroxylase, and aminopyrine N-demethylase activities were sexually differentiated in winter flounder, and in both species these activities were correlated to some extent with isozyme P-450A levels, suggesting co-regulation with or catalysis by P-450A. Altered microsomal monooxygenases in gonadally mature females accounted for several sex differences in metabolism, implying monooxygenase regulation by E(2). In immature winter flounder, E(2) injections shifted some microsomal activities and enzyme levels toward female-type patterns. This suggested that E(2) can regulate monooxygenase activities in winter flounder, and that additional factors regulate individual cytochrome P-450 isozymes in adult females.