The present situation of persistent xenobiotics in the environment and the associated adverse effects on human health and ecological systems has increased the need for an understanding of their sublethal effects in developing organisms. The developing embryos of medaka (Oryzias latipes) have been demonstrated to serve as valid models for endocrine disruption by xenobiotics and carcinogenesis. Unique features of these vertebrates make them particularly useful as subjects for developmental toxicity. The transparent chorion and the fact that they live as individual units free from the mother facilitate their use in determining when and where major developmental changes take place. The coupling of nuclear magnetic resonance (NMR) spectroscopy and somatic structure data provide precision and detail not obtainable without great time and expense in organisms whose embryo/fetal periods are intra uterine. This study will determine sub-lethal stress responses in developing medaka fish during embryogenesis. First, in vivo P-31 NMR Spectroscopy will be used to qualitatively and quantitatively determine the developmental stages at which the energetic biomarkers phosphocreatine and inorganic phosphate are present. Second, embryos will be exposed to a model toxicant to uncouple mitochondrial oxidative phosphorylation, such as trifluoro-methyl nitrophenol (TFM), or block the electron transport chain, such as rotenone, while energetic biomarkers are observed with NMR. Finally, these exposed embryos will be preserved for detailed cellular and tissue analysis in order to identify areas of apoptosis, necrosis, cellular differentiation or dedifferentiation. The perfusion system used in this study for evaluating biochemical changes by P-31 NMR in developing medaka embryos will also be used for assessing the potential developmental toxicity of complex aqueous mixtures. These studies should not only provide a better understanding of the changes in high-energy phosphagens of developing medaka embryos, but may be a valuable addition to the present knowledge of sublethal responses to toxicant exposures and their implications in human health and vertebrate embryo development.