Lactating mammary tissue from farm animals and small mammals was perfusion-fixed, prior to histochemical procedures, in an effort to localize the ouabain-sensitive Na+/K+-stimulated ATPase enzyme with the use of specific inhibitors. Histochemical evidence suggests that the Na+/K+-stimulated ATPase is located predominantly on the cytoplasmic side of the plasmalemma in myoepithelial and secretory cells of the alveolus and in epithelial cells of the ducts. Enzyme is not present on the apical plasmalemma of the secretory or duct cells. This substantiates the previous suggestions of localization of the sodium pumps on lateral and basal surface in lactating mammary secretory cells and duct cells.