The long-range goal of the proposed research is to better understand mechanisms whereby foreign organic chemicals produce malformations and other forms of toxicity in developing human embryos. Research on cultured whole rodent embryos has provided evidence that endogenous, embryonic, P450-dependent bioactivating systems will catalyze the conversion of otherwise inactive chemicals to reactive intermediates at rates sufficient to effect grossly observable morphologic abnormalities in the selfsame embryos. Evidence for several P450 isozymes in various tissues of rodent embryos has now been obtained and one of these (P450IA1) has been positively identified and partially characterized. Because almost nothing is known concerning the complement of P450s in human embryonic tissues during the critical stages of organogenesis (days about 20-60 of gestation) and because highly sensitive techniques are now available for such studies, this proposal is designed to detect and characterize the complement of P450 isoforms in tissues of organogenesis-stage human embryos. A combination of highly sensitive probes will be utilized to characterize functionally active P450s, immunologically cross-reactive isoforms and corresponding mRNAs in various human embryonic tissues. Information obtained from human tissues will be compared to that obtained from rodents and other experimental animals in an effort to determine which embryonic species may most closely resemble human embryos. The generation of reactive intermediates by cultured whole rodent embryos will also be compared with the capacity of cultured human embryonic tissues to generate the same or similar metabolic intermediates. Investigations of modes/mechanisms of regulation of human embryonic P450s will also be conducted. Relevance of the obtained information will be probed in experiments with cultured whole embryos. The feasibility of the proposed approaches has been demonstrated in preliminary experiments. Results obtained from the research should provide health professionals and regulatory agencies a more rational basis for providing advice and counsel to pregnant women with respect to their exposure to drugs and other foreign organic/environmental chemicals.