Dsx and mab-3-related transcription factor (DMRT1), Protein kinase C inhibitor (PKCIW) and Female-specific expression transcription factor (FET1) have been regarded as critical candidates of avian sex determination. Their mRNA expression was analyzed in chicken embryos during experimentally induced female-to-male sex-reversal by an aromatase inhibitor (AI) fadrozole. DMRT1 expression was higher in male (ZZ) gonads than in female (ZW) gonad prior to and throughout the period of sex differentiation. However, female-to-male sex-reversed ZW embryos showed elevated levels of DMRT1 expression similar to those of normal males, indicating that DMRT1 was associated with testis development. PKCIW gene expression was dimorphic between male and female gonads, and was up-regulated in AI-treated female embryos. This finding might account for the specific effect of PKCIW, functioning via heterodimerization with PKCI during avian sex determination. However, its elevated expression appeared to be insufficient to induce ovary development. On the other hand, FET1 expression was female-specific and unchanged in AI-treated female embryos. Results of present experiment suggested that DMRT1 is a determinant for testis development in birds.