Serial post-ictal serum prolactin levels were obtained over a period of one hour after bilateral or unilateral electroconvulsive therapy (ECT) in six patients. Bilateral ECT yielded significantly higher mean post-ictal prolactin levels than unilateral ECT at the 15, 20, 30 and 60 minute sampling times. These findings demonstrate a greater hypothalamic-stimulating effect of bilateral than unilateral ECT, and may explain the reported therapeutic advantage of bilateral over unilateral ECT in the treatment of patients with melancholia.