More than 20 years have passed since the discovery of the gene SRY (Sry in mice genome) which turn on the process of mammalian sex determination. Factor SRY is the key trigger of this process by means of initiation the autosomal gene SOX9 expression. SOX9 transcript is the master regulator of Sertoli cell differentiation and the male type of gonad development. In this review the features of SRYand SOX9 expression, intracellular transport and the targets of SRY and SOX9 transcripts, the male and female signal pathways interaction and the mechanisms of initial gonadal sex differentiation are discussed. The evolutionary aspects of the role of SRY and SOX9 are also compared which implies the differentiation of the testes in males is conserved in vertebrates and SOX9 shows highly conserved as the master factor in this process.