TAYLOR et al.1recently described a new, unusually potent, non-hydrazine, monoamine oxidase inhibitor: pargyline hydrochloride (MO-911). Horwitz and Sjoerdsma,2in 9 hospitalized hypertensive subjects, observed marked orthostatic hypotension and a minimum of side-effects with this compound and suggested that extensive clinical trials were warranted. The latter investigators also pointed out that none of the numerous monoamine oxidase inhibitors previously studied had become valuable therapeutic agents for hypertension, either because of low potency or prohibitive toxicity. The purpose of the present study is to evaluate the hypotensive changes of pargyline hydrochloride in ambulatory hypertensive outpatients in relation to long-term control of blood pressure values and to compare them with those observed with sulphonamide diuretics. Pharmacology Pargyline hydrochloride is a member of a different chemical series from the hydrazide type of monoamine oxidase inhibitors in current clinical use. Chemically, it is N-benzyl-N-methyl-2-propynylamine hydrochloride and is readily soluble but unstable in