Germ cells were isolated from XX ovaries and XY and XX Sex-reversed (Sxr/+) testes of mouse embryos 14-16 days post coitum, and the activity of an X-chromosome-coded enzyme, hypoxanthine phosphoribosyl transferase (HPRT), relative to an autosomal one, adenine phosphoribosyl transferase was determined. The ratio of enzyme activities in XX Sxr/+ prospermatogonia was significantly higher than that in XY prospermatogonia, up to 2-fold, suggesting that the silent X chromosome is reactivated in XX male germ cells before birth, as it is in female germ cells. The ratio was several times higher still in XX oocytes than in XX prospermatogonia, confirming that the increase in HPRT activity reported in oocytes is only partly due to an X-chromosome dosage effect.