We investigated the effect of blocking endogenous LH-RH, by injection of anti-LH-RH sera, on serum prolactin levels previously elevated by treatment with haloperidol for 5 days. An acute intravenous injection of rabbit anti-LH-RH serum significantly reduced serum LH levels and blocked the hyperprolactinemia induced by haloperidol. In another group of rats, sheep anti-LH-RH serum induced a significant decrease of serum LH and also lowered serum prolactin levels previously elevated by haloperidol. In ovariectomized rats, sheep anti-LH-RH serum markedly reduced serum LH levels and also decreased serum prolactin elevated by the pretreatment with haloperidol. It is concluded that the blocking of endogenous LH-RH action results in a decreased release of prolactin in response to stimuli like haloperidol. These results support the possible existence of a paracrine interaction between the pituitary gonadotrophs and lactotrophs.