Early mammalian embryos of both sexes have bipotential gonads. In female embryos, supporting cell precursors in these structures differentiate into granulosa cells and an ovary is formed. In male embryos, transient expression of the Y-linked sex-determining gene Sry in the supporting cells promotes Sertoli cell differentiation and testis development. But when does Sry expression switch these cells from the female to the male pathway? By examining gonad development in a mouse line that carries a heat-shock-inducible Sry transgene, Hiramatsu and colleagues show, on p. 129, that Sry has to be expressed during a 6-hour time window immediately after its expression normally begins in XY gonads to induce testis development in XX gonads. Sry action during this unexpectedly short period, they report, is essential for the switch between female- and male-specific FGF9/WNT4 signalling patterns. These results provide new insights into gonadal sex determination and also define for the first time the critical time window in which a master gene that determines organ fate has to act.