The testes are essential endocrine regulators of fetal masculinisation and male development and are, themselves, subject to hormonal regulation during gestation. This review focuses, primarily, on this latter control of testicular function. Data available suggests that, in most mammalian species, the testis goes through a period of independent function before the fetal hypothalamic-pituitary-gonadal axis develops at around 50% of gestation. This pituitary-independent phase coincides with the most critical period of fetal masculinisation. Thereafter, the fetal testes appear to become pituitary hormone-dependent, concurrent with declining Leydig cell function, but increasing Sertoli cell numbers. The two orders of mammals most commonly used for these types of studies (rodents and primates) appear to represent special cases within this general hypothesis. In terms of testicular function, rodents are born "early" before the pituitary-dependent phase of fetal development, while the primate testis is dependent upon placental gonadotrophin released during the pituitary-independent phase of development.