Both pinealectomized ewes and ewes with pineals were castrated during the breeding season and the level of luteinizing hor-mone (LH) was measured in their sera and pituitaries by radioimmunoassay. The levels of LH in the serum of each ewe rose following castration and fluctuated widely from day to day, within the same day, and between different ewes castrated on the same day. The concentration of LH in the pituitaries rose to high levels 30 days after castration, indicating that not only did castration cause the pituitary to secrete more LH but also more LH was synthesized and stored in the pituitary. Continual infusion of 2 mg of melatonin daily did not affect the time of the peak of LH in serum before ovulation or the incidence of ovulation as compared to infusion with physiological saline. There was a significant increase in LH in serum 15–30 days after castration over levels obtained 1–13 days after castration. Infusion of melatonin into castrated ewes prevented this observed significant rise in non-i...