Selective inhibition of rat fetal 3β-Hydroxy-Δ5-steroid dehydrogenase with certain substrate analogs has produced a precise animal model of human congenital adrenal hyperplasia due to a genetic deficiency of this enzyme system. Inhibition of fetal testicular dehydrogenase dehydrogenase by the analog produces testosterone-reversible blockade of penis formation and of nipple anlagen resorption in male fetuses. These observations have been duplicated by maternal administration of rabbit antisera to testosterone-3-bovine serum albumin. Inhibition of the fetal adrenal dehydrogenase system leads to excess dehydroepiandrosterone (DHA) production which, in turn, produces corticosterone-reversible virilization of female fetal genitalia and nipple anlagen. Inhibition-induced excess DHA production may also explain the persistence of wolffian duct structures in male fetuses since recent studies indicate presence of a ductal dehydrogenase system capable of transforming DHA into testosterone and dihydrotestosterone. The striking properties of these analogs, e.g., tight binding to the enzymatic active sites, potency, specificity, prolonged duration of action and intrauterine effectiveness before differentiation of the fetal enzyme, can be ascribed to active site-directedirreversible (ASDI) inhibition. Application of principles of ASDI inhibition has led to the discovery of new ASDI inhibitors with a differing specificity of enzymatic inhibition and sites of action. Steroidal excretion patterns determined by gasliquid chromatography – mass spectrometry produced by these inhibitors support the sites of action determined in vitro. These ASDI inhibitors and hormone-antibodies give promise of highly specific and precise molecular probes of the chemical controls of sexual differentiation.