This study investigated the effects of daily photoperiod length and morphine on thyrotropin-releasing hormone (TRH) and prolactin secretion in sheep. Two groups of adult ewes were kept under either 16hL:8 h D or 8 h L:16 h D photoperiods for approximately 60 days. Then the photoperiods were reversed and approximately 60 days later push-pull cannulae were implanted into the hypothalamic stalk-median eminence. After a 7-day recovery period, the ewes were subjected to hypothalamic perfusion and blood was collected at 15-min intervals from the jugular vein. Perfusates also were collected into 15-min fractions. The first 3 h of perfusion served as an equilibrium period. During the next 2 h, saline was infused into one jugular vein. This was followed by a 2-hour morphine infusion (1 mg·kg–1 • h–1). Data from 13 ewes were analyzed for effect of photoperiod and drug on TRH concentrations in the perfusates and prolactin in the serum. Prolactin was significantly (p < 0.01) higher under 16 h L:8 h D than 8 h L:16 h D and was greatly increased (p < 0.001) by morphine infusion. TRH only tended to be higher (p = 0.05) under 16 h L:8 h D than under 8 h L:16 h D, but morphine infusion induced a rapid and highly significant (p < 0.01) increase in TRH secretion. There were no photoperiod and drug interactions for either TRH or prolactin. These results suggest that increased prolactin secretion induced by long-day photoperiods is not mediated by TRH, whereas the morphine-induced increase in prolactin secretion appears to be due at least partially to increased TRH sec