Electrical stimulation of rat posterior lobes in vitro inhibited bioactive corticotropin (ACTH) release from the intermediate lobe and promoted the release of corticotropin-releasing factor(s) (CRF). Both effects were calcium dependent. Released posterior lobe CRF was inactivated by thioglycolate, and the CRF activity could be accounted for by vasopressin. Results suggest strongly that vasopressin is the predominant CRF released from neurohypophysial axons, and that intermediate lobe ACTH release is submitted to an inhibitory control.