Serum prolactin was measured in lightly etherized rats between 4 and 5 a.m. and between 11 a.m. and 12 noon at various times after operation in intact rats and rats that had been operated on. Serum prolactin was elevated in the late nocturnal samples from intact rats and this elevation was prevented by pinealectomy. With time following the operation, daytime serum prolactin was also lowered in pinealectomized animals. In animals rendered anosmic by olfactory bulb removal, serum prolactin was also lowered. Superimposed pinealectomy only lowered late nocturnal serum prolactin in anosmic rats. Blinding the rats by ocular enucleation lowered nocturnal serum prolactin initially and dampened the daily rhythm in prolactin, but superimposed pinealectomy had no effect on the blinded animals. The combined sensory deprivation produced by both blinding and olfactory bulb removal resulted in low prolactin titers regardless of whether blood samples were taken during the day or late at night and, as in the case of blinded animals, there was no effect of pinealectomy. Superior cervical ganglionectomy also lowered serum prolactin. Serum prolactin declined following castration in intact animals and this decline was accentuated by pinealectomy. Either blinding or anosmia also lowered serum prolactin in the castrated animals but, as in intact animals, pinealectomy had little effect on the sensory-deprived rats. A further lowering of prolactin occurred in castrated animals which were both blinded and anosmic; in these animals, pinealectomy again had no additional effect. Measurement of the pituitary prolactin content in the castrated animals at sacrifice revealed a significant lowering of values in blinded animals, with restoration of the values to control levels by pinealectomy. Anosmia also lowered the pituitary prolactin content