The electrophysiological properties of the cell membrane of rat spermatogenic cells were studied using the whole-cell variation of the patch-electrode voltage-clamp technique. In late primary spermatocytes and early spermatids isolated from adult testis, a transient inward current followed by a slowly developing outward current was produced when the membrane potential was made more positive than -60 mV. Early spermatogenic cells which consist of spermatogonia and early spermatocytes were isolated either from new-born rats (12-14 days old) of from adult cryptorchid rats 15-21 days after the operation. In early spermatogenic cells, some showed a slowly developing outward current with negligible initial inward current, while others showed a recognizable inward current followed by the slowly developing outward current. The inward currents are identified as Ca2+-carried current, since replacement of external Ca2+ with Mn2+ reversibly diminished the current whereas Ba2+ or Sr2+ substituted for Ca2+. The reversal potential of the outward current changed from -65 to -12 mV when [K+]o was raised from 5 to 100 mM. The outward current was independent of [Ca2+]o and was blocked by tetraethylammonium chloride. Thus the current was identified as membrane-potential-dependent K+ current. During spermatogenesis from spermatogonia to early spermatids, the density of Ca2+ current increased while the K+ current density decreased significantly.