Studies were undertaken to investigate the nature of receptors involved in the prolactin-releasing action of histamine in male rats. The increase of plasma prolactin levels induced by third-ventricle injection of histamine was blocked by intraventricular injection of ranitidine, an H2-antagonist, but not by systemic administration of mepyramine, an H₁-antagonist. The H2-histamine agonists 4-methylhistamine and Dimaprit, given intraventricularly in unrestrained and ether-anesthetized rats, enhanced prolactin release. The effect of 4-methylhistamine was dose-dependent, whereas Dimaprit had opposite effects depending on the dose. In low doses, Dimaprit decreased whereas in higher doses it increased plasma prolactin levels. The stimulatory effects of both agonists, similar to those produced by histamine itself, were blocked by metiamide (H2-antagonist), but not by intraventricular mepyramine. High doses of mepyramine only partially decreased the effects of 4-methylhistamine. Ranitidine was able to prevent the prolactin response to 4-methylhistamine. The selective H₁-histamine agonist 2,2-pyridylethylamine had no action on prolactin release. 2-Methylhistamine, which exhibits predominantly H₁-mediated actions, increased the release of prolactin. Its effect, however, was blocked by low doses of metiamide and was obtained at higher concentrations than 4-methylhistamine. Mepyramine prevented 2-methylhistamine action only at high doses. It is concluded that the increased release of prolactin evoked by histamine in male rats is mainly due to its action on H2-receptors. In addition, the results altogether indicate that H₁-receptors have not a significant participation.

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