Aturban (phenglutarimide) is a neuropsychiatric agent, was used as therapeutic drug for parkinsonism. Phenglutarimide hydrochloride possesses parasympatholytic activity and has been available as an antiparkinson agent since its preparation by Tagman, Sury & Hoffmann (1952). Phenglutarimide is a muscarinic acetylcholine receptor antagonist. There are three subtypes of enantiomers of phenglutarimide. The affinity of the enantiomers of phenglutarimide at three muscarinic receptor subtypes was examined in vitro using field-stimulated rabbit vas deferens (M1 receptors) and guinea pig atria (M2 alpha receptors) and ileum (M2 beta receptors). Extremely high stereoselectivity was observed and higher affinities (up to 6000-fold) were found for the (+)-S-enantiomer. The stereoselectivity ratios were different at the three subtypes, and the stereochemical demands made by the muscarinic receptors were most stringent at M1 receptors. (+)-(S)-Phenglutarimide was found to be a potent M1-selective antagonist (pA2 at M1 = 8.53). Its receptor selectivity profile is qualitatively similar to that of pirenzepine. (-)-(R)-Phenglutarimide showed no comparable discriminatory properties.