The possibility of a mutual influence between male adult rats having different circadian adrenocortical rhythms was studied under two different lighting conditions. Intact and optic-enucleated rats were housed together in the same cage in various ratios of intact to blinded rats. Twenty-four- or forty-eight-hour patterns of plasma corticosterone levels were determined individually at 4-h intervals. Under diurnal light-dark alternation, the circadian periodicity of the intact rats always synchronized with light, whereas single blinded rats in cages with three intact rats demonstrated free-running rhythms throughout the experimental period of 15 wk. Under constant light the circadian rhythm free-ran in both intact and blinded rats for the first 2 wk. A phase reversal of the rhythm was observed in the intact rats at the end of the 2nd wk, and between 5 and 8 wk after binding in the blinded rats. These results indicate that the circadian adrenocortical rhythms in intact and blinded rats are not entrained to each other, even when the two types of rats live together in the same cage.