Sry is the Y-chromosomal gene responsible for initiating the pathway of male development in mammals, presumably by regulating downstream target genes. As a basis for examining the role played by Sry and other candidate sex-determining genes, we have used reverse-transcriptase PCR to produce a precise temporal profile of expression of Sry in the developing gonad. Sry expression first occurs at 10.5 days postcoitum (dpc) and is upregulated to reach a maximum level from 11.25 to 12 dpc. Expression is then steadily downregulated from 12.25 to 13.25 dpc and is completely extinguished by 13.5 dpc. Two genes implicated in sexual development are Amh, encoding anti-müllerian hormone (AMH), and Fra1, encoding Fos-related antigen-1. Expression profiles of Amh and Sry in fetal gonads indicate a possible role for AMH in the downregulation of Sry expression. We show, however, that the timing of Sry downregulation is unaffected in Amh-deficient mice, excluding a role for AMH as a negative regulator of Sry. Further, we have examined the possibility that SRY may activate Fra1 during testis determination by analysing the expression of Fra1 in mouse fetal gonads. Fra1 is not expressed at any stage at or around the critical period of sex determination when Sry transcripts are present, thus excluding a role for Fra1 in sex determination and differentiation.