The effects of restricting daily water intake to either a 30-min morning or evening period were investigated on: (1) the overall circadian pattern of adrenocortical activity, and (2) adrenocortical activity during the 20-min period immediately after water presentation. Morning water restriction produced a substantial increase in pre-water levels of circulating corticosterone without affecting evening levels, thereby changing the form of the circadian pattern. In contrast, evening water restriction did not elicit any pre-water increase in corticosterone levels, and there was no change in the circadian pattern. Both morning and evening water restricted animals showed a pronounced and rapid decline in corticosterone levels during the 20-min post-water period, with levels dropping by more than 50% within 10 min of water presentation. The dissociation of the pre-water elevation and post-water decline in corticosterone evident in evening water animals indicates that these 2 processes may be governed by different factors. It is suggested that the pre-water effect involves circadian regulatory mechanisms whereas the post-water effect is associated with the process of reinforcement.