Sixteen patients underwent electrophysiologic studies before and after intravenous infusion of propranolol, 0.1 mg/kg. All of them had intraventricular conduction disturbances. A significant (p less than 0.01) increase occurred after propranolol in the mean spontaneous cycle length (mean change 122 ms) and the AH interval (mean 28 ms). The HV interval did not change significantly. The effective refractory period of the His-Purkinje system increased in 4 of 5 patients (mean net change 44 ms, p greater than 0.10). In 7 patients with phase 3 infranodal block, the longest cycle length for producing block increased after propranolol in 5 and remained unchanged in 2 (mean increase 40 ms, p less than 0.02). Two patients showed phase 4 atrioventricular block. Propranolol increased the critical cycle length by an average of 120 ms. Propranolol-induced bradycardia restored stable atrioventricular conduction in 2 patients with intermittent 2:1 block but facilitated the occurrence of high grade infranodal block in 1 patient. Thus, propranolol can impair His-Purkinje conduction in patients with preexisting infranodal abnormalities.