Suction ablations of the medial or lateral fornix were performed in order to transect selectively the medial corticohypothalamic tract (mcht) which originates in the anteroventral subiculum and travels in the lateral fornix terminating in the basal hypothalamus. The circadian rhythmicity of plasma adrenal corticosteroid levels was assessed in individual animals 1--2 weeks postoperatively. Ablation of the lateral fornix disrupted the periodicity of corticosteroid secretion which is normally synchronized with the light--dark cycle, whereas medial fornix ablation or neocortical ablation caused no such disruption. Group mean levels of plasma adrenal corticoids were higher in the lateral fornix-ablated animals than in the medial fornix-ablated, neocortically ablated, or intact control animals. These findings suggest that the anteroventral subiculum is important in the regulation of adrenal corticosteroid rhythmicity, and that it exerts an inhibitory influence upon corticosteroid release.