The study has been approved by the Helsinki Committee and all patients must give written informed consent. Patients will be evaluated for study if they meet DSM-IV criteria for major depression without psychotic features. Patients can enter the study if they had at least 3 weeks of treatment with SSRI at clinically adequate doses (150mg fluvoxamine, 20mg fluoxetine, or 20mg paroxetine) and still have a score of at least 18 on the Hamilton Depression Scale (HDS) (on 24 item scale) with at most mild improvement from onset of SSRI treatment. Patients with alcohol or drug abuse or patients with unstable medical illnesses are excluded. Design is parallel double-blind. Patients continue SSRI treatment of the same medication and dose they received before entry. They are randomly assigned to phenytoin or identical placebo capsules. Patients will be rated on the HDRS weekly for four weeks after addition of phenytoin or placebo. Patients who do not improve at least 1 unit on the GCI every two weeks will be dropped from the study for ethical reasons and referred to open treatment. HDRS ratings are done weekly by an experienced psychiatrist (BN) blind to the treatment medication.