Nialamide produces effective monamine oxidase inhibition resulting in a three‐ to fivefold increase in tryptamine excretion when administered in a dose of 100 mg. for 10 days. L‐Dihydroxyphenylalanine, given in a dose of 75 to 100 mg. to normal volunteers receiving nialamide in a dose of 100 mg. daily for 10 days, produces a prompt and prolonged elevation in the blood pressure. Flushing of the face, heat, and pounding of the heart were poduced in most subjects. The blood pressure elevation was considered to be secondary to the intake of large amounts of the potent precursor in the presence of effective monamine oxidase inhibition. The active amine was not identified but could have been dopamine, norepinephrine, or a mixture of the two.