Ouabain-sensitive Na+-K+-ATPase activity was localized histochemically in the submandibular gland of the mouse under various conditions using p-nitrophenylphosphate as substrate at pH 9. In untreated adult males and females, intense staining was seen in the basally striated portions of the epithelial cells lining the excretory and striated ducts. The region of the lateral cell membranes, but not of the apical plasmalemma, also stained. In granular convoluted tubules (GCTs), strong staining was seen only in a narrow band of the basalmost region of the cells; in males this stained region was thinner than in females, and frequently was absent. The baso-lateral margins of acinar and intercalated duct cells gave a very weak reaction. In untreated males, or in females that were treated with dihydrotestosterone, overall staining for the enzyme was always less than in untreated females, due to the diminished reactivity of androgen-stimulated GCT cells and the decreased number of striated ducts. However, in females treated with triiodothyronine, enhanced activity of Na+-K+-ATPase was indicated by stronger staining in all cell types, including the hypertrophied GCT cells. Na+-K+-ATPase activity was undetected in the submandibular glands at birth, but moderate staining was seen in the larger excretory and striated ducts by 5 days of age. From 10 days of age onward, intense staining was seen in the excretory and striated portions of the ramifying duct system. Developing GCT cells could not be distinguished from their precursor cells in the striated ducts until 25 days of age. These data indicate that the salt-handling capacity of the submandibular gland of the mouse varies with both endocrine status and age.