Two bull calves were subjected to changes in daily illumination over a 12-week interval, with ambient temperature maintained at 22 C. When photoperiod was shortened from 16 to 8 h, serum prolactin decreased from a maximum of 57 mg/ml to a minimum of 8 ng/ml. Conversely, with another 4 bulls, increasing the photoperiod from 8 to 16 h of light caused serum prolactin concentrations to increase from 25 to 100 ng/ml. The increase in serum prolactin in response to increasing photoperiod was delayed approximately 7 weeks, whereas the response to decreasing photoperiod was delayed only about 1 week. Changes in photoperiod had no effect on serum LH concentrations. We conclude that changes in photoperiod account, at least partially, for the seasonal changes in serum prolactin previously noted in cattle.