VIP stimulated prolactin secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of GH, LH, FSH was not affected. The stimulation of prolactin was dose-dependent, with an apparent affinity of VIP of 10.9 ± 3.1 nM and a maximal stimulation of 57.7 ± 4.2%. Secretin, a structurally related peptide, was also active at higher concentrations whereas another partial analogue, glucagon, was ineffective. The effect of VIP was not blocked by α-flupentixol, a potent dopaminergic antagonist, at concentrations which antagonized the dopamine inhibition of prolactin secretion. Stimulation by VIP and TRH was additive. Neither Metenkephalin nor naloxone interfered with the response to VIP. It thus seems that specific VIP receptors are present on pituitary prolactin cells. VIP, present in the mediobasal hypothalamus and detected in the hypothalamo-hypophyseal portal blood therefore is a good candidate as a physiological P