1 Sex differentiates under genetic control during successive periods. Classical morphological and experimental data have shown the sexual bipotentiality of the developing structures. But, as a matter of fact, several observations indicate that both sexes are not equal or equipotential as to their developmental trends and mechanisms. 2 The developmental analysis of the body sex characteristics reveals a hormonal control. In animal experiments made by the author and by others it has been observed that many structures or systems develop along the feminine type in the absence of testes during several critical developmental stages. These structures include the genital tract, the hypothalamic centres controlling the pituitary function, the nervous structures mediating sex behaviour and possibly other tissues. The ovary is unnecessary for the feminine differentiation of these structures; in males, femaleness has to be repressed and maleness imposed by the testes. 3 The problem of gonadal sex differentiation is re-evaluated; developmental aspects occurring during normal development or in the gonads of freemartins in cattle are examined. During early sexual differentiation of the gonads, testes rapidly differentiate whereas ovaries are first characterized mainly by the fact that they do not become testes. These observations can be interpreted by assuming that in males a signal imposes masculinity on the gonadal primordia which otherwise would slowly become ovaries. 4 It is hypothesized that throughout sexual differentiation in mammals, maleness has to be actively imposed on a system which will become feminine if it escapes this control.