AbstractA study of sex differentiation of the Wolffian and Müllerian ducts of fetal guinea pigs was made by an organ culture method, and the morphogenetic changes of the ductsin vitrowere compared with thosein vivo. A total of 19 reproductive tracts were explanted from fetuses 29‐30 days old. The explants were cultured for nine days, with or without gonads. The Wolffian ducts degenerated in male tracts cultured without testes and in female tracts with or without ovaries, but they were maintained in male and female tracts cultured with one or two testes. The Müllerian ducts degenerated in all male tracts; in female tracts with or without ovaries they enlarged and coiled extensively and they were not inhibited when testes were cultured with the female tracts. It is concluded that in the guinea pig fetus at 29‐30 days of age, the Wolffian ducts of both sexes have become androgen‐dependent and that they are normally maintained in the male by testicular androgen. The Müllerian ducts of males were conditioned to degenerate before the time of explantation. The Müllerian ducts of females were not dependent upon ovarian hormone for their maintenance and de