Sertoli cell-gonocyte cocultures obtained from rat testes 20 days postcoitum, 1 day postpartum, and 3 days postpartum were used to investigate the effect of FGF-2 on both somatic and germ cells in vitro during the perinatal period. With cells isolated from fetal, newborn, or 3-day-old animals, FGF-2 was found to significantly increase the number of Sertoli cells after 3 or 6 days of cultures, starting at a concentration of 1 ng/ml. FGF-2 did not increase the [3H]thymidine labeling index of Sertoli cells, indicating that FGF-2 is a survival factor for these cells in vitro. FGF-2 (1, 5, or 10 ng/ml) also significantly increased the number of gonocytes after 6 days of culture with cells from either newborn or 3-day-old animals. About twice as many germ cells were found in those cultures compared to the control cultures. Addition of a neutralizing antibody against FGF-2 to control cultures caused a significant decrease in the number of gonocytes compared to that in untreated cultures after 6 days, whereas with FGF-2, the antibody decreased the number of germ cells to control levels. FGF-2 significantly stimulated the proliferative activity of the gonocytes after 3 or 5 days, indicating that FGF-2 is a survival as well as a mitogenic factor for these cells. Taken together, these data suggest that FGF-2 is an important factor around the start of spermatogenesis, at least in vitro.