Young male Wistar rats were maintained under controlled light schedule (light:darkness 12:12 h). Control group had free access to food, while other two groups were fed either for 2 h at the beginning of light period (morning hours) or for 2 h at the beginning of dark period (evening hours). The levels of serum and adrenal corticosterone in a control group were increased during the light period and decreased during the dark one. Circadian rhythm of corticosterone level was not remarkably changed in a group fed at evening hours. However, in a group fed at morning hours the level of corticosterone increased at the end of a dark period, while it decreased after the intake of food. It was concluded that the time of feeding appears to be an important circumstance modifying a circadian rhythm of serum and adrenal corticosterone in rats.