The mechanism of biosynthesis of estrogens in vitro is under study. Optimum conditions will be determined and the role of cofactors, various buffers, inhibitors and intermediates will be examined. The transport of steroids across the placental membrane is being investigated. The rate of diffusion and metabolic alterations will be determined for representative steroids. The role of the intestinal flora of biliary steroids is being examined with emphasis on 21-dehydroxylation of corticoids.