Two distinct hyperpolarizing responses are produced when histamine is iontophoretically applied onto the somal membranes of identified neurons within the cerebral ganglion of Aplysia: a biphasic response consisting of a rapid component (less than 5 sec) usually superimposed upon a slowly developing component; or a monophasic slowly developing response 5-20 sec in duration. The reversal potential values for the fast (typically -65 mV) and the slow (typically -89 mV) responses, and their shift to new values when the external potassium or chloride concentrations were altered, revealed that the fast and slow potentials are produced predominantly by conductance increases to chloride and potassium ions, respectively. The effects of histamine H1- and H2-receptor agonists and antagonists were studied to characterize the pharmacological properties of histamine receptors mediating these two ionically dissimilar hyperpolarizing responses. The slow potassium-dependent hyperpolarization could be mimicked by several histamine analogues; the most potent tested were the H1-receptor agonist, 2-methylhistamine, and the H2-receptor agonist, 4-methylhistamine. Neither of these agents mimicked the fast chloride-dependent histamine response. The slow potassium-dependent responses induced by histamine or histamine agonists were completely and reversibly blocked by the H2-receptor antagonist, cimetidine. By contrast, the slow potassium-dependent hyperpolarizations produced by iontophoretically applied acetylcholine or by dopamine to the same neurons were unaffected by cimetidine. Other H1 and H2 antagonists tested were either ineffective, or only partially blocked the slow hyperpolarizations in a non-selective manner. The fast chloride-dependent hyperpolarizations were not selectively antagonized by any of the H1 or H2 reagents tested, although they were effectively suppressed by tubocurarine and strychnine. These data indicate that two pharmacologically distinct histamine receptors mediate potassium- and chloride-dependent hyperpolarizations in Aplysia neurons. Neither of these receptors, however, could be classified as strictly H1 or H2 according to criteria presently used in non-neuronal tissues. The selectivity and reversibility of cimetidine indicate that this particular antihistaminic could be a valuable pharmacological tool for defining putative histaminergic synapses in Aplysia and perhaps other nervous systems.