Ten patients with cardiac conduction defects due to severe tricyclic antidepressant poisoning were successfully treated with intravenous phenytoin. Eight patients had combined first degree AV block and intraventricular conduction delay; one patient had first degree AV block alone; and one patient had intraventricular conduction delay alone. Phenytoin was injected at a rate of 50 mg/min in amounts not exceeding 500 mg (approximately 5 mg/kg to 7 mg/kg). Five patients had complete normalization within 46 min. The remaining five patients showed improvement in their conduction defects shortly after phenytoin treatment, with complete normalization occurring within 14 hr. This investigation demonstrated the effectiveness of phenytoin in reversing tricyclic-antidepressant-induced cardiac conduction abnormalities. Phenytoin is a logical drug for the treatment of severe tricyclic antidepressant poisoning signalled by first degree AV block and/or intraventricular conduction delay.