Septal influences on resting adrenocortical activity were tested in the pigeon by electrical stimulation or electrolytic lesions located in either the medial or the lateral septum. Stimulation of the medial septal area in unanesthetized unrestrained pigeons with chronic implants led to a marked decrease in plasma corticosterone. Stimulation of the lateral septal area was not effective in promoting any corticosterone variation. Lesion placement in the nucleus septalis lateralis resulted in a moderate increase in basal corticosteronemia. The elevation in circulating corticosterone was much higher after destruction of the n. septalis medialis and high values were found at every time during the day. Suppression of the diurnal variation in plasma corticosterone with such a high stable level was compared to previous data which were obtained in pigeons after hippocampectomy and after partial anterior or complete deafferentation of the hypothalamus.