Cytoplasmic content of histochemically demonstrable glycogen in secretory cells of eccrine glands of the rat appeared directly related to the sodium state. On the other hand, histologic and ultrastructural alterations consisting of prominence of clear cells, cytoplasmic vesiculation, and an increase in cytosomal bodies and Golgi structures correlated with variations in water balance. Correlation of light and electron microscopic findings disclosed that the optically clear spaces observed in eccrine glands by the former technique and frequently referred to as vacuolization represent secretory cells with a lucent cytoplasm and paucity of organelles rather than a specific structural aberration. Despite certain differences, it is concluded that the eccrine apparatus of the rat may represent a suitable model for the study of certain parameters of the sweating phenomenon.