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Four a subunit isoforms have been identified in the human V-ATPase: al, a2, a3, and a4. Subunit al is expressed ubiquitously; subunit a2 was detected in the kidney, lung, and spleen; and subunit a3 was localized in osteoclasts, where it is essential for bone resorption. Subunit a4 was detected in the kidney, inner ear, and the murine epididymis. Two d subunit isoforms have been identified for the human V-ATPase: dl is ubiquitous, whereas d2 is present in kidney, lung, and osteoclast. Recent studies suggested that the diverse physiological functions of the V-ATPase in different membranes are established through the utilization of specific subunit isoforms. Selective interactions between these different isoforms were proposed to govern the subcellular V-ATPase targeting and to determine whether the pump will be located either in intracellullar structures or in the plasma membrane. Our laboratory has shown expression of subunits Bl, B2, and E2 in narrow and clear cells of the rat and human epididymis and vas deferens. More recently, localization of subunit a4 was reported in the murine epididymis. Staining of narrow and clear cells in the mouse epididymis was also obtained using an antibody raised against the 67-kDa subunit (subunit A) of the V-ATPase expressed in the fungus Neurospora crassa. However, very little is known about the localization of other subunits of the V-ATPase in the epididymis and vas deferens, and the present study is aimed at characterizing the cellular and subcellular localization of subunits A, C1, C2, G1, G3, a1, a2, a4, d1, and d2 in epithelial cells of these organs.