Pinealectomized male hamsters underwent complete testicular regression in response to injections of melatonin given 3x/day at three-hour intervals. The responsiveness to melatonin appeared to be the same regardless whether the compound was administered at 0900-1500h, 2000-0200h or 0100-0700h. This result is different from that observed in pineal-intact hamsters, where malatonin is relatively ineffective if given at 0900-1500h. From these results we conclude that the diurnal rhythm in sensitivity to melatonin observed in the intact hamster is probably related to some aspect of pineal activity and is not an "intrinsic" component of the circadian system.