To test the possibility that the septum is essential forrhythmic pituitary-adrenal function, septal nuclei and fibers were re-moved by suction from adult female rats. Rhythmic pituitary-adrenal function was assessed by fluorometric measurement of non-stress plasma corticosterone in blood samples obtained at different times during the 24 h light-dark cycle. At 3 and 5 weeks after surgery, a.m. and p.m. non-stress plasma corticosterone levels were comparable in all groups, and all groups showed significant diurnal variations in steroid levels. Diurnal steroid patterns were studied in individual rats at 9 weeks after surgery by collecting 3 blood samples (0.5 ml) at 10 h intervals (07.00, 17.00, 03.00). Septal ablation, with concomitant interruption of fornix, was compatible with normal diurnal variations in plasma corticosterone concentrations. Lesioned rats demonstrated vaginal cycles and normal numbers of tubal ova at estrus. The data indicate that the diurnal pituitary-adrenal rhythm persists in the absence of septum and fornix and suggest that these structures are not essential for this rhythm.