In a group of 288 patients with acute inferior (diaphragmatic) myocardial infarction, second and third degree atrioventricular (AV) block was diagnosed in 37 (14%). Three of the 37 died. The AV block in the 34 survivors could be differentiated into two distinct types, namely, early and late AV block. In 15 patients, second and third degree AV block developed within 6 hours of the first signs of infarction. In these 15 patients, all signs of AV block disappeared within 24 hours after infarction. Second and third degree AV block appeared suddenly in the vast majority, and first degree AV block could be detected in only a few patients and for a very short time before normalization of conduction. Atropine either abolished AV block completely or caused a marked acceleration of ventricular escape rhythm. In 14 patients, second and third degree AV block developed later than 6 hours (in 12 later than 24 hours) after infarction. It was heralded and followed by relatively long periods of first degree AV block in all cases (except in two patients who were admitted 72 hours after infarction). The total duration of AV block was longer than 40 hours in all of these patients, and the ventricular rate was relatively high. In no patient was abolishment of AV block achieved by atropine, and ventricular acceleration was relatively slight in all. In five patients, early and late AV block could be recognized consecutively. The two types of AV block seem to have different causes. Increased vagal tone is probably operative in the first type, and metabolic changes due to ischemia in the second. Response to atropine and sympathomimetic drugs is much better, and cardiac pacing only rarely indicated, in patients with early than in those with late AV block.