The microinjection of vasoactive intestinal peptide (VIP) into the medial preoptic area rapidly inhibited pulsatile luteinizing hormone (LH) secretion within 1 h in ovariectomized rats. A transient but significant increase in prolactin (PRL) secretion occurred after VIP injection coincidentally with the initiation of the inhibition of LH pulses. Preoptic microinjections of secretin or peptide histidine isoleucine amide (PHI) did not have such effects. These results suggest specific actions of VIP, despite its structural similarity to secretin and PHI, on the preoptic mechanisms governing LH and PRL secretion in ovariectomized rats.