Vasa deferentia of 20 human adult males and 8 children were divided into four portions (proximal, middle, distal and terminal) for histological, histochemical and ultrastructural studies. The epithelium of the adult vas deferens was formed of columnar and basal cells. Three types of columnar cells were observed: (1) principal cells with PAS-positive cytoplasmic granules (lysosomes); (2) dark or pencil cells with a dark cytoplasm and nucleoplasm and cytoplasmic lipid vacuoles, and (3) mitochondrion-rich cells with numerous mitochondria similar to those of principal cells. All these cell types showed stereocilia and irregular convoluted nuclei, some of them containing granular, PAS-positive intranuclear inclusions. Principal cells were the most frequent cell type in the proximal portion but their number gradually decreased along the length of the vas deferens, while the proportion of both pencil cells and mitochondrion-rich cells increased. The muscular coat comprised three muscular layers: (1) inner longitudinal, (2) middle oblique (circular in the distal and terminal portions), and (3) outer longitudinal. The functional significance of regional variations is discussed. The infant vas deferens showed a columnar cell type only. However, cells either with numerous lysosomes or with numerous mitochondria were occasionally found. No regional variations were observed in the infant vas.