The enantiomers of three 5-methyl-2-pyrrolidone analogues of the muscarinic agent oxotremorine (1) were synthesized. The pyrrolidine derivative (R)-13 was an antagonist to carbachol in the guinea pig ileum and also showed central and peripheral antimuscarinic activity in vivo. It was more potent and more selective than atropine in antagonizing the central effects of 1. The dimethylamino analogue (R)-14 and the trimethylammonium salt (R)-15 were potent agonists in the guinea pig ileum. (R)-14 showed both central muscarinic (hypothermia) and central antimuscarinic activity (antagonism of oxotremorine-induced tremor) in vivo. The R enantiomers of 13-15 were considerably more potent than the S enantiomers in vivo and in vitro irrespective of whether agonist or antagonist activity was measured. From a comparison of the contribution of the methyl group at the chiral center to the overall affinities, it is suggested that agonists and antagonists in this series bind in an essentially identical manner to the muscarinic receptor.