The selectivity of 4-methylhistamine (4-MH) as an agonist at histamine H1- and H2-receptors has been evaluated in the guinea-pig isolated ileum. The EC50 values of 4-MH on H1- and H2-receptors that mediate contractile responses were determined. The EC50 at H1-receptors was estimated after selective blockade of H2-receptors by tiotidine and the EC50 at H2-receptors estimated after selective blockade of H1-receptors by mepyramine. The -log EC50 values at H1- and at H2-receptors were 4.57 and 5.23, respectively. The dissociation constants for the interaction of 4-MH with H1- and H2-receptors were determined. The -log KD values at H1- and H2-receptors were 3.55 and 4.27, respectively. These results suggest that 4-MH is only about 5 times as potent at H2- as it is at H1-receptors in the guinea-pig ileum and that 4-MH should be used with caution to discriminate between H1- and H2-receptors.