Mouse gonadal tissue was studied under various conditions of in vitro culture, with the aim of clarifying some of the somatic-cell influences that regulate the development of germ cells in the mammalian gonad. Gonadal ridges, with or without the adjacent mesonephric region, were removed from mouse embryos 10.5-12.5 days post coitum (dpc). In an organ culture system, the female ridges showed good development, with no masculinization. All germ cells entered meiosis at the expected time. Although some oocytes entered the growth phase, many primordial follicles were observed. 11.5- and 12.5-day male ridges formed testis cords, and the germ cells developed as T-prospermatogonia. In 10.5-day ridges, cells resembling Sertoli cells differentiated but did not form testis cords, and the germ cells entered meiosis. We conclude that full differentiation of the supporting cell lineage was not achieved when culture was begun at 10.5 dpc; our findings suggest that immature Sertoli cells neither form testis cords nor inhibit the entry of germ cells into meiosis. When the ridges were fragmented and cultured in gas-permeable dishes, the somatic cells grew out as a monolayer on which the germ cells rested. Under these conditions male germ cells did not enter meiosis and did not survive for more than a few days. Female germ cells entered meiosis. In contrast to the organ culture system, many of the surviving oocytes entered the growth phase during the second week of culture, reaching diameters of up to 60 microns. This suggests that normal follicular cell investment may play a crucial role in maintaining the oocyte in a state of developmental arrest. The growing oocytes showed the oocyte-specific expression of the enzyme glucose phosphate isomerase. It seems that the initiation and maintenance of both oocyte growth and oocyte-specific gene expression can take place in the absence of normal follicular cell investment.