Electrical stimulation of hypophysial posterior lobes in vivo evokes a significant decrease of plasma growth hormones (GH) and an increase of plasma corticotropin (ACTH) concentrations. Electrical stimulation of posterior lobes in vitro evokes the simultaneous release of GH inhibiting factor(s) (GHRIF) and ACTH releasing factor(s) (CRF) into the medium. Pretreatment of media with thioglycolate abolishes the CRF and GHRIF activity, but reveals GH releasing factor(s) (GHRF). Median eminence extracts and vasopressin have potent GHRF and CRF activity. Vasopressin may account fully for the CRF and partially for GHRF activity. Results suggest that hypothalamo-neurohypophysial axons release GHRIF, vasopressin and possibly a GHRF into a portal circulation to modulate the secretion of GH and ACTH.