The effect of Vasoactive Intestinal Peptide (VIP) on plasma prolactin has been tested in two human subjects. When perfused at low doses (few micrograms per minute), VIP increased the prolactin levels in both sexes, the response being more important in Woman (3.5-fold over basal) than in Man (2-fold). VIP may be a physiological regulator of prolactin release in humans. This peptide may be a specific tool for the clinical investigation of the prolactin function.