An acute episode of symptomatic sinus bradycardia, occurred in a 50-year-old female patient after she had been given a single therapeutic dose of mianserin. Heart rate was corrected by atropine injection. Re-administration of mianserin resulted in the recurrence of bradycardia. Further examination showed no cardiac abnormalities. This case is the first report of conduction defect in a patient given therapeutic doses of mianserin.