The electrocardiographic effects of imipramine hydrochloride at therapeutic plasma concentrations were determined in 44 depressed patients during a 6-week clinical outcome study of depression. During each week of the protocol, i.e., 2 weeks of control and 4 weeks of drug treatment, a standard 12-lead ECG, high-speed, high-fidelity ECG tracings, and a 24-hour continuous ECG recording were obtained. PR, QRS, and QTc intervals, T-wave amplitude, heart rate and frequency of ventricular premature depolarizations (VPDs) were measured. The plasma concentration of imipramine and desmethylimipramine was measured three times a week. Imipramine prolonged the PR (p less than 0.001), QRS (p less than 0.001) and QTc (p less than 0.001) intervals, increased the heart rate (p less than 0.001) and lowered T-wave amplitude (p less than 0.05) during the 4 weeks of treatment. No patient developed high-grade atrioventricular block or severe intraventricular conduction abnormalities. In addition, imipramine had a potent antiarrhythmic action in patients who were recovering from depression. Ten of 11 patients who had more than 10 VPDs/hour had 90% or greater arrhythmia suppression during antidepressant treatment with imipramine at plasma concentrations ranging from 100--302 ng/ml.