When mares were pinealectomized in summer or in winter there was no difference in the annual ovulation date of these animals and of pineal-intact controls during the first post-operative breeding season but in the second season the ovulation date of the winter pinealectomized mares was significantly delayed, suggesting a long-term but clear pineal involvement in seasonal reproductive patterns. Exposure of pinealectomized mares to a stimulatory photoperiod failed to advance the average date of first annual ovulation and was similar to that of intact mares receiving no stimulatory lighting. The results indicate that pinealectomy blocks the ability to respond to photostimulation. In intact mares plasma melatonin concentrations were significantly increased during darkness (P less than 0 . 003) but pinealectomized mares demonstrated no significant time trends.