Prolactin-releasing (PRF) activity was found in Pitressin® (a commercial extract from posterior pituitary for vasopressin). Injection of Pitressin into conscious free-moving rats implanted with a permanent atrial indwelling cannula, produced a transient increase in prolactin concentration in the circulation. In order to find out whether the PRF activity was due to vasopressin or to an unidentified component in the Pitressin, we tested synthetic lysine vasopressin and demonstrated that vasopressin (1 U/kg) elevated plasma prolactin concentration about threefold. In contrast, oxytocin (1 U/kg) did not alter the prolactin concentration. In order to find out whether the effect of vasopressin is a direct or indirect action, we tested the vasopressin effect on hypophysectomized rats which had previously been implanted with 2 adenohypophyses under the kidney capsule. Again this dose (1 U/kg) of vasopressin elevated circulating plasma prolactin. These experiments indicate that vasopressin can elevate circulating prolactin concentration in nonestrogen-primed normal male rats and that vasopressin also stimulates prolactin secretion from transplanted glands dissociated from direct hypothalamic control.