The available evidence indicates that electroconvulsive therapy (ECT) has a marked and specific antimanic effect. Data on change in seizure threshold during ECT and acute effects of ECT on cerebral blood flow are presented in support of the view that the therapeutic effects of ECT are related to its anticonvulsant properties in manic patients. Specifically, an antimanic effect of ECT was associated with a significantly greater increase in seizure threshold during bilateral ECT and, across treatment conditions, with a significantly greater reduction in cerebral blood flow acutely following ECT. A low initial seizure threshold during bilateral ECT was found to be associated with a poor therapeutic response. This, along with earlier findings in depressed patients, suggests that heterogeneity of the disease processes underlying mood disorders may be critical for an understanding of the mechanisms of action of ECT.