The agonistic potencies of (±)muscarine, (±)cis-2-methyl-5-[(dimethylamino)methyl]-1,3 -oxathiolane methiodide (cis-oxathiolane) and its two enantiomers were determined at muscarinic M1 and M2 receptors in the pithed rat. In non-pretreated animals, i.v. administration of these agents produced bradycardic effects mediated by cardiac M2 receptors followed by increases in heart rate mediated by M1 receptors in sympathetic ganglia. As these responses have been shown to partly overlap, "true" M1 and M2 potencies were determined after selective blockade of M1 and M2 receptors by pirenzepine and methoctramine, respectively. A similar rank order of agonist potencies was obtained at M1 and M2 receptors: (+)cis-oxathiolane > (±)cis-oxathiolane > (±)muscarine > (-)cis-oxathiolane. At both receptor subtypes, (+)cis-oxathiolane was considerably more potent (ca. 30-lold) than its corresponding (-) enantiomer indicating that the agonist binding sites of the two receptor subtypes may have similar stereochemical properties. While (±)muscarine showed similar potencies at M1 and M2 receptors, racemic cis-oxathiolane and its two enantiomers showed a slight selectivity (3 - 7 fold) for M1 receptors indicating the potential usefulness of these compounds in the development of selective M1 receptor agonists.