A series of experiments were designed to determine the origin of a nerve growth promoting protein found in high concentration in the tubular portions of the submandibular glands of adult male mice. This protein is necessary for the growth and development of immature sympathetic nerve cells. The quantities of this protein can be increased in the glands of female mice to levels found in the submandibular glands of adult male mice by injecting testosterone into preadolescent or adult iemale mice. When testosterone was given to young mice or to adult female mice together with an antiserum prepared against the nerve growth factor ( N G F ) , the NGF content of the testosterone-treated mice increased in the same manner as in testosterone-and normal serum-treated mice. Although antiserum against the nerve growth protein could be demonstrated in the sera of mice during the course of the entire experiment, the antiserum had no effect on the content of NGF in the submandibular glands. After the intravenous injection of NGF in large quantities, it could not be localized in liver, spleen or submandibular gland cells of the mouse as determined by fluorescent antibody techniques. The results suggested that the nerve growth pro- moting protein found in high concentration in the submandibular glands of adult male mice and in testosterone-treated female mice is synthesized and stored in cells of the tubular portion of these glands.