In sheep embryos, steroidogenic activity has been reported as taking place during the period of sexual differentiation. In the case of mouse embryos, the sporadic detection or absence of steroidogenic enzymes suggests that the ovary is inactive. The purpose of this work was to establish if mouse undifferentiated gonads express steroidogenic enzymes in a similar way as in sheep embryos. To know this, we analyzed the mRNA expression pattern of 3β-Hsd1 and P450arom as well as protein expression pattern of 3β-HSD1 and Testosterone in normal undifferentiated and differentiated gonads from both male and female mice embryo. Our data indicate that there is expression of 3β-Hsd1 in XX gonads during gonad differentiation period. Nevertheless the Testosterone which would indicate steroidogenic activity is not produced. Besides, the absence of P450arom indicates that the production of Estradiol as observed in the ovaries of sheep does not occur. The detection of 3β-Hsd1 in the early stages of ovarian development, as well as the absence of Testosterone suggests that XX gonads are not steroidogenic and that 3β-Hsd1 enzyme may play a different role than in the steroidogenesis process.