The M2 subtype of the muscarinic receptor was investigated using the antagonist AF-DX 116. In "in vitro" and "in vivo" experiments, AF-DX 116 behaved as a competitive antagonist and exhibited a selectivity for cardiac muscarinic-mediated functions. It antagonized negative chronotropic and inotropic effects exerted by muscarinic receptor activation in the guinea pig atria with an affinity (pA2) 10-fold greater than that estimated in intestinal and tracheal smooth muscle preparations. It also reversed the negative chronotropic effect induced by peripheral vagal stimulation, intrasinus node injection of bethanechol and central vagal activation by clonidine. In all these in vivo functions, AF-DX 116 was approximately 10 times less potent than atropine, while being several hundred-fold less potent in antagonizing acetylcholine-mediated bronchoconstriction and gastric emptying. These results are consistent with the hypothesis that the cardiac muscarinic receptors constitute a subclass of the M2 subtype.