Monoamine oxidase (MAO) activity was assayed in various human tissues obtained at autopsy by the method of Lovenberg et al (Jour. Pharmacol, and Exptl. Therap. 135; 7, 1962). MAO activities are expressed as [mu]m of indoleacetic acid formed per g of tissue per hour of incubation. The averages of MAO activities were: liver 25, kidney 15, heart 10, lung 8.1, thyroid 6.4, adrenal 6.3, pancreas 4.9, sympathetic ganglion 4.4, cerebral cortex 4.0, pituitary 1.6, spleen, 1.0, skeletal muscle .69, and testis .49. In 13 normal subjects MAO activity was assayed in specimens of jejunal mucosa obtained by peroral biopsy. The averae MAO activity was 40.4 [plus or minus] 3.1 and the range was 32.3 to 46.2. It is suggested that this is a practicable approach to the direct measurement of tissue MAO activity in man.