In 21 patients suffering from endogenous depression and given nortriptyline 150 mg/day for 4 weeks in a once-a-day dosage an electrocardiographic study was made with ECG recordings at different times during the treatment. At the same time the plasma concentration of nortriptyline was determined. During the treatment the heart rate increased significantly, both at the minimum and the maximum concentration of nortriptyline. At the maximum concentration the PQ interval increased significantly. None of the patients developed an A-V block. 3 patients developed extrasystoles. None of the patients had severe subjective symptoms. All finished the trial as planned. There was no correlation between the plasma concentration of nortriptyline and the increase in heart rate. The trial revealed no serious cardiac influence during treatment with nortriptyline in a once-a-day dosage.