Melatonin secretion was investigated in ewes maintained in continuous darkness for 6 days and following an acute delay of lights off as part of a study offactors controlling the melatonin rhythm. In continuous darkness, the interval between successive offsets in circulating melatonin was always greater than 24 hr, indicating that the decline in melatonin secretion in sheep is controlled by endogenous mechanisms having a period longer than 24 hr. In ewes placed under extended darkness with no delay in the time of lights off, the decline in circulating melatonin was delayed by 2.5 ± 0.2 hr. Animals that had dusk delayed by 4 hr continued to secrete melatonin for at least 4 hr after subjective lights on. Under these conditions, it is proposed that the offset of melatonin secretion is influenced by the timing of lights off and the onset of melatonin secretion. The brain centers controlling melatonin secretion in sheep appear to operate in a manner similar to several species, and thus the sheep may prove to be a useful experimental animal for further studies of circadian mechanisms