This study enrolls patients who were treated for depression in an earlier research study and whose depression has returned since stopping treatment. Patients are treated for 4 months to bring the depression under control. Patients are then assigned randomly (like tossing a coin) to receive either the full dose of nortriptyline or half the usual dose of nortriptyline. Patients continue taking nortriptyline for 2 years or until a major depression returns. Throughout the study, patients are monitored for symptoms of depression and other side effects.