Spermatogenesis, a tightly regulated developmental process of male germ cells in testis, is associated with temporal and spatial expression of certain gap-junction connexins. Our findings by RT-PCR indicate that the Cx31 gene is expressed in testis tissue of adult and postnatal rats. During the postnatal spermatogenic process, the Cx31-specific signal became detectable at 15 dpp and onward by in situ hybridization, and apparently localized in the basal compartment of seminiferous epithelium where active spermatogonia and early primary spermatocytes reside. No signal was found in the luminal region. In adult testes, spermatids of elongation phase were also Cx31 positive. Immunohistochemical analysis with mouse anti-Cx31 antibody gave a similar staining pattern, providing further evidence that the gap-junction protein is abundant in the basal seminiferous epithelium, in accordance with the cellular distribution of Cx31 mRNA. These results represent the first demonstration of Cx31 expression at both transcriptional and protein levels in the seminiferous epithelium of rat testes. Thus, Cx31 may play a role in cell-cell communication during spermatogenesis.