Female rats maintained on a 23 h water deprivation schedule show an altered circadian rhythm of pituitary-adrenal activity. Significant elevations of plasma corticosterone are observed in the morning if the watering time is between 1000 and 1100 h. These elevations continue to early afternoon if watering occurs at 1500 h. The normal late afternoon peak is abolished. Different patterns of adrenocortical rhythms are observed when the watering time is shifted from morning to afternoon.