Male and female rats were treated with medroxyprogesterone acetate (17 alpha-acetoxy-6 alpha-methyl-pregn-4-ene-3,20-dione, MPA) 600 mg/kg body weight i.p. daily for seven days. The steroid metabolizing enzymes in liver microsomes and liver homogenate were measured following MPA treatment and in control rats. The specific activity of NADPH- and NADH-5 alpha-reductase in female rats and of NADPH-5 alpha-reductase in male rats decreased by the treatment. NADP+- and NAD+-3-hydroxysteroid dehydrogenase activities were lower in female MPA rats if compared to untreated animals. In male rats only NADP+- and NAD+-3 beta-hydroxysteroid dehydrogenase activities (substrate 5 beta-dihydrotestosterone) were diminished by MPA administration. There was no effect on the cytoplasmatic 5 beta-reductases. Plasma concentrations of luteinising hormone (LH), testosterone and androstenedione were lowered by MPA treatment.