Forty-eight 2½-year-old Merino rams were maintained in a specially insulated and ventilated enclosure at constant body weight on a constant nutritional level. Four were slaughtered each month and changes studied in the reproductive tract. Weight of epididymides, sperm motility and morphology did not vary but the weight of testes, seminiferous tubule diameter and number of epididymal sperm decreased as daylength increased. Seminal vesicle weight and fructose content were more unpredictable.