The electron microscopy of human eccrine sweat glands has been studied before and after stimulation by pilocarpine iontophoresis. The identity of the dark and clear cells in the secretory segment as defined by Montagna et al. (23) was determined by studying serial sections, thin for electron microscopy and thick for light microscopy. Cells with numerous apical secretory vacuoles are termed mucoid (dark) cells, since these vacuoles stain positively for acid mucopolysaccharide. Clear cells are intimately associated with intercellular canaliculi. The "cuticular border" of surface cells of the duct is a condensation of tonofilaments and granules. Numerous mitochondria are concentrated in basal cells of the duct. The presence of mucoid cells in the secretory segment may bear on the interpretation of the pathologic findings in the disease cystic fibrosis of the pancreas, and suggests that this disease may be due to a basic disorder of mucopolysaccharide production. The possible roles of the various cellular components in the elaboration of sweat are discussed.