Treatment of litter-mate rats with the antiserum to the mouse submaxillary gland nerve growth factor of Levi-Montalcini and Cohen resulted in significant decreases in monoamine oxidase activity of the superior cervical, stellate, and thoracic chain ganglia, the spleen, submaxillary glands, kidneys, and liver. The monoamine oxidase activity of the retinas, pineal gland, pituitary gland, lungs, atria, and uterus showed small but statistically insignificant decreases. The monoamine oxidase activity of the ventricles, various portions of the stomach, and proximal and distal small intestine was not affected by treatment with the antiserum. It is postulated that a relation can be shown between sympathetic innervation and monoamine oxidase activity. This can be shown only in those tissues in which the monoamine oxidase associated with sympathetic function constitutes an appreciable portion of the total monoamine oxidase activity. In most of the organs and tissues studied, only a small portion of the monoamine oxidase activity is associated with the nerve endings of the sympathetic postganglionic fibers, whereas the larger portion of the total activity is associated with nonadrenergic sites. Monoamine oxidase activity was measured by the method of Lovenberg and co-workers ( J. Pharmac. exp. Ther., 1962), with slight modifications found to give reproducible, quantitative results (Klingman and Klingman, Biochem. Pharmac., 1966).