Quantitative evaluation of antimuscarinic effects of antihistamines (H1- and H2-receptor antagonists) was carried out using a receptor-binding assay. The -inhibition constants (Ki values) of twenty seven H1-receptor antagonists, one related antidepressant and three H2-receptor antagonists at H1-receptors and muscarinic receptors in the bovine cerebral cortex were determined. All the H2-receptor antagonists examined showed very low affinity for the muscarinic receptors. On the other hand, some H1-receptor antagonists (mequitazine, cyproheptazine, clemastine, diphenylpyraline, promethazine, homochlorcyclizine and alimemazine) had high affinity for the muscarinic receptors (Ki = 5.0-38 nM). Another group of H1-receptor antagonists (mepyramine, terfenadine, metapyrilen, azelastine, hydroxyzine and meclizine) had low affinity for the muscarinic receptors (Ki = 3,600-30,000 nM). Thus, a broad range of antimuscarinic potencies among the antihistamines was demonstrated. These results should provide helpful information with regard to the clinical and experimental use of antihistamines.