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Two experiments were carried out to study the effects of controlled lighting régimes on plasma levels of LH, testosterone and prolactin in Romney rams. In the second experiment the rams were either pinealectomized or sham-operated so that the role of the pineal gland in mediating seasonal changes in reproduction could be examined.

Levels of testosterone and prolactin were considerably influenced by the lighting schedule. Peak plasma concentrations of testosterone were associated with periods during which the daily photoperiod decreased, whereas plasma levels of prolactin showed a pattern of changes approximately in phase with the lighting cycles. Mean plasma concentrations of LH were low in all groups of rams, which made the detection of significant effects of any treatment very unlikely. Pinealectomy reduced the effects of changes in the daily photoperiod on the patterns of secretion of testosterone and prolactin. These findings establish the pineal gland as an organ which influences the endocrine responses of rams to photoperiodic stimuli and it is concluded that the pineal gland is probably important as a mediator of seasonal reproductive changes in these animals.