Forty-two outpatients with major depressive disorder were treated with oral fezolamine in a 6-week, three-center open-label study. Therapy was initated at 100 mg/day; thereafter dosage was increased based on the response of the patient. Maintenance dosage usually ranged between 100 and 450 mg/day. Clinically significant improvement relative to the patient’s prestudy state was observed after 2 weeks in both patient and physician-rating scales. Fifty-five percent of patients improved their Hamilton Psychiatric Rating Scale for Depression (HAM-D) scores by more than 50%. The median dose associated with a clinically significant response was 245 mg/day. Five of the 6 patients who dropped out did so because of gastrointestinal adverse effects. The most common adverse effects were nausea (36%), headache (29%), constipation (26%), and dry mouth (24%