We studied the location of Sox9 protein in the embryonic, juvenile, and adult rat testis by immunohistochemistry and immunoblotting. Sox9 belongs to a family of Sox proteins that are transcription factors and important in several developmental processes. In the incipient embryonic testis, Sox9 was prominently present in the gonadal blastema. With further embryonic differentiation, Sox9-positive cells arranged in the periphery of the testicular cords, showing the location of the Sertoli cells. Thereafter the immunoreaction for Sox9 gradually declined and was only weakly detectable in the 2-day-old postnatal rat testis. This situation remained for some period of time. In the 15-day-old rat testis, Sox9 protein strongly reappeared in the testicular cords. In the adult, the Sertoli cells of most regions of the seminiferous tubules were positive for Sox9. The strongest reaction for Sox9 was found in the dark zone. However, clearly negative or only weakly positive spermatogenic stages for the protein were also found, as seen for example in the pale zone. In fertile 1-year-old rats this basic situation was still detectable. Analyzed rat ovaries were all negative for Sox9, showing the sex- specific nature of Sox9. The results showed that Sox9 protein is distinctly present in the developing and mature Sertoli cells, but that its presence and amount is dependent on the age and the spermatogenetic stage within the seminiferous tubuli. The prominent presence of Sox9 in the incipient testis and at puberty suggests that this protein is needed at important phases of aggregation and reorganization of the Sertoli cells. The age and stage- specific presence of Sox9 in the testicular cords and in the seminiferous tubules of the adult suggests that Sox9 also may have a pivotal role in germ cell differentiation.