The effects of fluoxetine and amitriptyline on the electrocardiogram (ECG) and systolic time intervals (STIs) were measured during a double-blind parallel-group study in depressed patients. 2. ECGs and STIs were measured after a 1 week placebo run-in, following 1 week's treatment with fluoxetine 40 mg daily or amitriptyline 100 mg daily, and then after 3 weeks' treatment with fluoxetine 60-80 mg daily or amitriptyline 150-200 mg daily. 3. Fluoxetine had no effect on the ECG or STIs at any dose. Amitriptyline 150-200 mg daily shortened the sinus cycle length by a mean of 12%, prolonged the PR interval by 8% and the QRS duration by 10%. Amitriptyline did not significantly alter the STIs.