To assess the influence of the hippocampus on ACTH secretion, plasma ACTH concentrations were compared in hippocampectomized and control rats under conditions of differing plasma corticosterone concentrations. In the PM, hippocampectomized rats had higher basal and 2-min ether-stress-induced ACTH concentrations than did cortex-removed controls. Basal PM plasma corticosterone concentrations were also significantly elevated in the hippocampectomized group. In the AM, there were no between-group differences. Adrenalectomy abolished the PM differences between cortical-control and hippocampectomized rats. PM hypersecretion of ACTH in the absence of the hippocampus suggests that this structure contributes an inhibitory component to the neural mechanisms regulating ACTH release. The fact that this increase in hormone concentrations is limited to the PM indicates that there is a circadian variation in hippocampal action.