Weanling rats received bilateral electrolytic lesions in the dorsomedial (DMH) or ventromedial (VMH) hypothalamic areas destroying primarily the dorsomedial (DMN) or ventromedial (VMN) hypothalamic nuclei. Sham-operated rats served as controls. Lesions in the VMN and DMN, both of which have previously been shown to disrupt normal diurnal feeding rhythms, were also observed to disrupt normal plasma corticosterone rhythms in the present study. The a.m. values of plasma corticosterone in the DMN-lesioned rats were higher than the sham-operated controls. In the p.m., the values of both VMN- and DMN-lesioned rats were lower than those of the controls but unchanged in comparison to their own a.m. concentrations. This disruption of the normal diurnal plasma corticosterone rhythm persisted for at least 9 post-operative weeks.