The effect of immunoneutralization of endogenous vasoactive intestinal polypeptide (VIP) and peptide histidine isoleucine (PHI)-like peptides by administration of specific and potent antisera to the respective peptides on ether stress-induced prolactin (PRL) release was examined in male rats bearing an indwelling atrial catheter. Forty-five minutes after an injection of 1 ml of either normal rabbit serum (NRS), anti-VIP serum (AVS), anti-PHI serum (APS) or AVS plus APS, the rat was placed for 1 min in a beaker containing an ether-impregnated cotton ball. Ether exposure caused a prompt and significant increase in plasma PRL in all of the animal groups tested. Pretreatment with either antisera did not affect basal plasma PRL levels, whereas plasma PRL rises after ether exposure were significantly lower in rats pretreated with AVS, APS or AVS plus APS than those with NRS. These results suggest that hypothalamic VIP and PHI-like peptides may be involved, at least in part, in the mechanism by which ether stress stimulates PRL secretion in rats.