Quiescent gonocytes were isolated from fetal testes of rat 18-day post coitum and cultured alone or on monolayers of somatic cells from different origins. The gonocytes specifically adhered to Sertoli cells, isolated from 21 to 23-day-old rat testes; this adherence was necessary for their survival in vitro. Addition of follicle-stimulating hormone and testosterone to these cultures did not increase the viability of the gonocytes. Serum was found to be deleterious to the germ cells. Electron-microscopic examination of Sertoli-cell-gonocyte co-cultures revealed the presence of numerous adhesion plaques between these cells, indicating that Sertoli cells and gonocytes are able to communicate in vitro. Gonocytes, in co-culture with Sertoli cells, were viable for at least 9 days. The gonocytes did not spontaneously resume proliferation. The simple culture system described in the present paper should be useful in studying the nature of the factors that are responsible for sending the quiescent gonocytes into the cell cycle and for stimulating the formation of A spermatogonia, a process characterizing the start of spermatogenesis.