The hormonal control of rhythmic fluctuations in hepatic microsomal mixed function oxidase activity was examined by incubating microsomes prepared from intact control, sham‐operated, pinealectomized (PX), adrenalectomized (AX), and pinealectomized‐adrenalectomized (PXAX) adult male rats with aminopyrine (AP) and [4‐C14]3ß‐hydroxy‐5‐androsten‐17‐one (DHA). The animals were kept on a 12:12 light‐dark cycle for 15 days prior to killingatl, 4, 6 and 10 hafter light and at 1, 4, 6 and 10 h after dark. The rate of both theN‐demethylation of AP and the 7α‐hydroxy lation of DHA varies diurnally with maximal and minimal activities in the dark and light periods, respectively. The fluctuation in both reactions fits a symmetric sine curve for all endocrine treatment groups except for the 7α‐hydroxylation in PX animals. PXAX was the only treatment resulting in a significant increase in 7α‐hydroxylase activity. The data suggest that the pineal may be involved in mediating the effects of environmental light on liver 7α‐hydroxylase activity.