Fetal mouse testes and ovaries with their urogenital connections were cultured singly or in pairs on Nuclepore filters. When a testis in which the sex was not yet morphologically detectable was cultured together with older ovaries containing germ cells which were progressing through the meiotic prophase, the male germ cells were triggered to enter meiosis. When older fetal testes in which the testicular cords have developed were cultured together with ovaries of the same age with germ cells in meiosis, the oocytes were prevented from reaching diplotene stage. It was concluded that the fetal male and female gonads secrete diffusable substances which influence germ cell differentiation. The male gonad secretes a "meiosis-preventing substance" (MPS) which can arrest the female germ cells within the meiotic prophase. The female gonad secretes a "meiosis-inducing substance" (MIS) which can trigger the nondifferentiated male germ cells to enter meiosis.