[Circadian photosensitivity and photoperiodic control of the annual cycle of blood prolactin in the mink].

An annual cycle in plasma prolactin was evidenced in male Minks with a minimum in fall and a maximum in June. In two series of night interruption experiments, it was found that the photoperiodic control of the prolactin cycle involved a circadian rhythm in sensitivity to light whose photosensitive phase culminated 16 hrs. after dawn. The same photosensitivity rhythm also induced the annual testis cycle, although, in the latter case "long days" had an inhibitory effect instead of the stimulatory control for prolactin.