We investigated the effects of histamine applied by microiontophoresis onto serotonin-containing (serotonergic) cells recorded extracellularly in the dorsal raphe nucleus of the rat. Application of histamine at low iontophoretic currents (1-5 nA) produced a rapid depression of the firing of all serotonergic neurons tested. The H1-receptor antagonists mepyramine and diphenhydramine were unable to attenuate the histamine-induced response. Antagonism of the effect of histamine by the iontophoretic application of the H2-receptor antagonists cimetidine and metiamide was not possible to evaluate since both were found to exert potent inhibitory effects by themselves. In contrast, the nonimidazole-derived H2-receptor antagonist ranitidine, which had no effect by itself, selectively antagonized the histamine-induced depression of neuronal activity. Histidine, 3-methylhistamine and a variety of histamine agonists selective for H1- or H2-receptors were unable to mimic the effect of histamine in dorsal raphe. Histamine's effects may, in part, be mediated at a gamma-aminobutyric acid receptor complex as the gamma-aminobutyric acid antagonists bicuculline and picrotoxin rapidly and reversibly antagonized both the histamine- and the cimetidine-induced depression of serotonin cell firing; the glycine antagonist strychnine selectively blocked the inhibitory effect of glycine without altering the histamine-induced response. These data show an inhibitory effect of histamine on serotonin-containing neurons in the dorsal raphe; this effect may be partially mediated at a subtype of H2-receptor. These data further indicate that the inhibitory effects of histamine and cimetidine observed in the dorsal raphe nucleus may result, in part, from an action directly or indirectly at a gamma-aminobutyric acid receptor complex.