The degree of A-V block increased after intravenous administration of atropine in 10 nondigitalized patients with acute inferior myocardial infarction who had narrow QRS complexes during periods of 1:1 A-V conduction. Short episodes of 3:1, 4:1 and 5:1 A-V block were seen to emerge: (a) in 6 patients, directly from Wenckebach periods; (b) in 3 patients, from alternating Wenckebach periods; and (c) in 1 patient, from a 3:2 Wenckebach period which led to a short-lived alternating Wenckebach period. Apparently, the predominance of the chronotropic effects on the sinus node over the dromotropic effects on the A-V node led to a tachycardia-dependent (more ischemic than vagal) process, exposing or producing multi- (two, three or four) level block involving the A-V node (and perhaps the His bundle). Subsequently, therapeutic pacing was instituted in 9/10 patients because they developed spontaneous symptomatic advanced A-V block. Therefore, it is possible that the early effects of atropine identified a narrowly-defined subset of patients in whom prophylactic pacing may be indicated. However, more studies are necessary to corroborate these assumptions.