Although the electrocardiographic effects of the tricyclic antidepressants have been extensively investigated, there are fewer data on the effects of monoamine oxidase inhibitors and tetracyclics on cardiac conduction. This study used high speed recordings of the electrocardiogram to investigate the cardiographic effects of phenelzine and mianserin and to compare these to the effects of imipramine, amitriptyline, and placebo. Phenelzine caused significant slowing of the heart rate, while mianserin showed little effect on heart rate compared to the increases in rate seen with tricyclics. In clinically effective doses, neither phenelzine nor mianserin caused changes in conduction, while both tricyclics studied caused the expected prolongation of conduction. These data suggest that phenelzine and mianserin deserve further study in patients with disease of the cardiac system as they may be less likely to cause heart block in these patients.