Effects of food restriction and its withdrawal on the circadian adrenocortical rhythm were studied in adult female rats under constant darkness (DD) and constant lighting (LL). The animals were fed restrictedly during the daytime under DD and at night under LL for 10 days, and thereafter allowed to take food ad libitum. All the studies were performed as group data. The circadian peak of mean plasma corticosterone (B) levels was demonstrated just before the beginning of feeding under either DD or LL condition. Thus, the food restriction seemed to be an apparent single entrainer of the periodicity of plasma B levels if the light-dark cycle was absent. However, the circadian variation of plasma B was rapidly shifted after the withdrawal of 10-day food restriction and turned to a similar pattern to those seen in the rat fed ad libitum throughout DD or LL for 10 days. The circadian periodicity of food intake after the withdrawal of food restriction was also shifted to the pattern shown by the rat fed ad libitum under DD. These findings suggest that the forced alteration of eating rhythm induced by the restriction of feeding may act only as a temporary entrainer of the periodicity in the pituitary-adrenal system. Thus, it is possible that an endogenous adrenocortical rhythm may exist under the free-running conditions and only be entrained by feeding schedules, and that the food restriction regimen may have no essential effect on the supposed biological clock(s) which regulates the circadian rhythm of the pituitary-adrenal system as well as that of locomotor activity or eating behavior.