The human adult intraepidermal eccrine sweat duct (IESD) were studied by the transmission electron microscope. The formation of the intracytoplasmic cavities was demonstrated in the luminal cell of the adult IESD, and the relation between the intracytoplasmic cavities and the IESD formation was as follows. On the eccrine sweat duct ridge, these intracytoplasmic cavities enlarged, broke through the cell membrane and coalesced into the intercellular lumen. Furthermore, several intercellular lumina were merged into the intraepidermal ductal lumen. In the lower to upper squamous layers, small intracytoplasmic cavities were formed around the intraepidermal duct and were fused into the duct. Simultaneously, the contents of the intracytoplasmic cavities were discharged into the ductal lumen. Intracytoplasmic cavities in these layers were ordinarily formed and concerned in not only the reconstruction of the duct but also the secretory function. The intracytoplasmic cavities were surrounded with the same microvilli as lined the intraepidermal ductal lumen. Many clear vesicles (about 0.1 microns in diameter) were observed near the intracytoplasmic cavities, but multivesicular dense bodies (MDB) were not seen near the cavities. These vesicles were considered to participate in the formation of the intracytoplasmic cavities.