The effects of fluoxetine on the ECG were compared to those of placebo, imipramine, amitriptyline, and doxepin. ECG tracings at the beginning and end of several drug studies were evaluated retrospectively, without knowledge of the drug to which patients had been assigned. Mean changes in heart rate revealed a statistically significant decrease in the fluoxetine group. Active control drugs increased heart rate: increases were significant for imipramine and amitriptyline but not doxepin. Intraventricular conduction delays were noted in 5 patients who received imipramine and 1 patient who received amitriptyline: 4 of these patients developed left bundle branch block. No intraventricular conduction delays were noted in fluoxetine-treated patients.