Tranylcypromine is a safe and potent mono-amine oxidase inhibitor which, when used in combination with sedatives, produced improvement in 74% of 84 ambulatory depressed patients of both psychotic and psychoneurotic types. The onset of benefit was rapid, occurring within an average of 7-9 days. The improvement was more rapid and more dramatic when the drug was combined with trifluoperazine. The incidence of significant side effects was low. No case of blood dyscrasia or jaundice occurred in our series. Some of these cases have been followed for over one year and have had no relapse.