Electroconvulsive therapy (ECT) effects on monoamine transmitter metabolites in the cerebrospinal fluid (CSF) were evaluated in three patients after completion of a course of bilateral or unilateral ECT. Each patient had earlier undergone an unsuccessful trial with an antidepressant drug. Despite the disparate nature of the basic pharmacology of the antidepressant drugs used, common chronic effects were observed in the CSF, with reductions in 3-methoxy-4-hydroxyphenylglycol (MHPG) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in all patients despite lack of therapeutic response. Homovanillic acid (HVA) changes were inconsistent. After ECT, however, no CSF changes were observed in the one nonresponder to that treatment. The two ECT responders showed marked increases in CSF 5-HIAA and HVA over their respective baselines, with an elevation in MHPG in one patient only. Further study of the mechanisms of action of ECT should focus on the serotonin and dopamine systems and on the differences between responders and nonresponders.