Atrioventricular and intraventricular conduction were assessed in 4 patients with tricyclic antidepressant overdosage. The proximal or A-V nodal conduction (A-H) was normal, but 3 patients showed a drug-related increase in the QRS duration and His-Purkinje (H-V) conduction time. One patient with a normal H-V interval had a normal width QRS. The findings were independent of the heart rate and suggested a "quinidine-like effect" of the tricyclic drugs when ingested in suicidal doses. All 4 patients had normal intracardic conduction when restudied 8 days later. Procainamide and quinidine have similar effects on the intracardiac conduction and would appear undesirable in the treatment of cardiac arrhythmias in patients with tricyclic overdosage. Practolol in a dose of 20 mg by intravenous injection reduced the heart rate and caused a relative increase in the A-H interval but did not affect either the distal atrioventricular or intraventricular conduction. Although there was a slight drop in blood pressure, it may be a safer drug to use.