Summary:An assessment of the side effects of therapeutic doses of tricyclic anti‐depressant drugs was attempted in 32 patients with depressive illness. The patients studied had no evidence of clinical heart disease or hypertension and were not receiving any other drugs.Moderate increase in heart rate and mild prolongation of atrioventricular conduction occurred. No significant effect on the corrected QT interval or blood pressure was found. There was no correlation between the increased heart rate, prolongation of the atrioventricular conduction time (PR interval) and plasma nortriptyline levels measured in 20 out of 32 patient