The Leydig cells, responsible for testicular androgen production, have two growth phases during the life-span of mammals. The fetal population appears during fetal life and is responsible for the androgen-induced differentiation of the male genitalia. The fetal Leydig cells disappear after birth, and the other population, the adult Leydig cells, appears during puberty and persists for the whole adult life. The fetal Leydig cells, evidently due to the intrauterine endocrine milieu and their special functional requirements in genital differentiation, differ both morphologically and functionally from the adult population. The purpose of this review is to elucidate the special features of the mammalian fetal Leydig cell population, which presents an intriguing experimental model for studies of function and regulation of steroidogenic cells.