For Drosophila, the choice between male and female development is made by the switch gene, Sxl, in response to the X:A ratio. Once Sxl is turned on in females, it actively maintains the determined state, independent of the X:A signal, by a positive autoregulatory feedback loop in which Sxl proteins direct the female-specific splicing of Sxl transcripts. In this paper we have investigated the mechanism controlling pathway initiation. Our results suggest a two-step model for the initial activation of Sxl in females. In the first step, a special class of Sxl mRNAs is expressed in female embryos from an early promoter that responds to the genes signaling the X:A ratio. The proteins produced from these early mRNAs then initiate the autoregulatory loop by directing the female-specific processing of transcripts from the late Sxl promoter.