Certain members of the cytochrome P-450 family are expressed at different levels in the livers of male and female rats. Although little is known of the functional significance of these sex differences, progress has been made towards the understanding of the endocrine control of hepatic sex differences in cytochrome P-450 levels. Jan-Ake Gustafsson and colleagues describe a subpopulation of hepatic sexually differentiated P-450s that is regulated by sex differences in growth hormone (GH) secretory pattern. This secretory pattern is in turn regulated by gonadal steroids. These studies demonstrate a novel action of GH and suggest that the hormonal secretory rhythm is pivotal in determination of biological effects.