Mouse primordial germ cells (PGCs) arrive at the urogenital ridge (UGR) at around 10.5 days postcoitum (dpc). They proliferate until around 13.5 dpc, then enter into meiosis in the female or become mitotically arrested in the male gonads. In this study, meiotic transition of mouse PGCs was examined in vitro. Female PGCs obtained from UGRs or genital ridges at 10.5-11.5 dpc began to express meiosis-specific genes, Scp3 and Dmc1, after dissociation and cultivation on feeder cells for several days. Meiotic transition into the leptotene stage was confirmed by the formation of axial cores. Male PGCs at 10.5-11.5 dpc and migratory PGCs obtained from mesenteries at 10.5 dpc also expressed Scp3 and formed axial cores after several days of culture, supporting the hypothesis that PGCs are capable of entering meiosis before arriving at the UGR. gp130-mediated signaling, known to promote survival/growth of PGCs and also to inhibit the differentiation of embryonic stem cells, suppressed the expression of Scp3 in PGCs and inhibited the following formation of axial cores in vitro. This novel activity of gp130-mediated signaling may provide some clues for the understanding of pluripotency of mammalian germ-line cells and/or the sex differentiation of fetal germ cells.