AbstractThe plantar sweat glands of the rat have been studied by light and electron microscopy. These glands differ from conventional eccrine sweat glands of man, other primates, and cats in that the secretory segment contains only one type of secretory cell. This cell is characterized by a relatively nondescript cytoplasm containing numerous vesicles, extensive interdigitations of adjacent plasma membranes and small microvilli projecting into the lumen. In the resting gland, two types of cells can be distinguished on the basis of mitochondrial morphology. Some cells contain mitochondria in condensed configurations, and others contain mitochondria in the more orthodox configuration. Following stimulation, these differences are not pronounced; and the condensed mitochondria have evidently changed into more orthodox mitochondria in appearance. No distinct cytologic pathway for secretion could be defined, correlated with the most unusual ionic composition of the sweat in this animal (very high levels of potassium). The sweat glands of the rat foot pad then are unique as compared with other cutaneous glands both in their cytologic appearance and in the chemical composition of the sweat they elaborate.