Patients, ages 18 to 80, with a diagnosis of major depression (without psychotic features), will be randomized to double-blind treatment outpatient study to receive either memantine (5-20mg/day) or placebo for a period of 8 weeks. Following this acute period, patients who fully respond could enter a 16-week continuation phase. Acute efficacy will be determined by demonstrating a greater response rate using specified criteria. Approximately 112 patients with acute major depression will be enrolled in the study.