An autoradiographic study of nuclear estrogen binding was performed in developing human urogenital sinuses and vaginas derived from first and second trimester specimens. Nuclear estrogen binding was detected in all specimens greater than or equal to 10 weeks of gestation within mesenchymal cells. Nuclear labelling within epithelium was observed only in those specimens whose development and differentiation was advanced. Thus, mesenchyme appears to be the initial estrogen target tissue within the developing human vagina and may play a fundamental role in estrogen-induced teratogenesis of the human genital tract.