The histamine-induced relaxation of bethanechol-contracted rabbit trachea was analyzed pharmacologically in experiments designed to determine whether this tissue contains relaxant histamine H2-receptors or an atypical histamine receptor. Relaxation to histamine was potentiated by inhibition of histamine H1-receptors with chlorpheniramine. A cimetidine-resistant relaxant effect of high concentrations of histamine was observed which correlated with concomitant decreases in organ bath pH upon addition of histamine. Neutralization of the histamine solution eliminated the cimetidine-resistant effects of histamine. Cimetidine, metiamide and burimamide were competitive antagonists of histamine-responses with pKB values of 6.6, 6.1 and 5.4, respectively. These estimates are not significantly different from literature values for antagonism of H2-receptors by these drugs in guinea-pig right atria. The potency ratio of histamine and 4-methylhistamine (a selective H2-receptor agonist) was not significantly different from that obtained in rabbit right atria, an H2-receptor-containing tissue. These data from studies with antagonists and agonists indicated that histamine produced relaxation of rabbit trachea by activation of histamine H2-receptors.