PURPOSE: We attempted to apply the microgravity cell culture system for rat testicular germ cell maturation in vitro. METHODS: Primary spermatocytes were isolated from immature male rat by sedimentation velocity. Sertoli cells were isolated from another immature male by enzyme digestions. Sertoli cell aggregates were plated into conventional tissue culture flasks and incubated at 37 degrees C for 48 hours. These pretreated Sertoli-enriched monocultures were used in preparing Sertoli cell-primary spermatocyte cocultures. And then, primary spermatocytes and Sertoli cells were cocultured in a microgravity cell culture device for 28 days. RESULTS: Cell viability rate is more than 50 % after a 28-day long period of incubation. Furthermore, about 23 % haploid germ cells are observed. CONCLUSIONS: These results using primary spermatocyte coculture with Sertoli cell aggregates under microgravity show that it is possible to mature these cells up to the round spermatid and even to elongating/elongated steps. It may be possible to overcome the male sterility due to maturation arrest at the primary spermatocyte stage.