Summary of Work: In vitro fertilization provides a unique opportunity to isolate and observe certain human reproductive processes, and thus potentially to differentiate among possible mechanisms of reproductive toxicity. In this clinical protocol, the woman undergoes pharmacologic hyperstimulation of her ovaries, and multiple ova are surgically extracted and fertilized in a dish, usually with her husband's sperm. Thus, the quality of ova is assessed, rates of fertilization and cleavage can be studied, early development of the embryos can be observed, and finally implantation into the uterus can be assessed, after the transfer of a known number of embryos. In collaboration with staff at UNC School of Medicine, we undertook a study to relate exposures to clinical outcomes in IVF. In necessary methods-development work, we have now extended our models for fertility to this clinical setting. The model we propose allows one to assess which characteristics of the embryos can serve as useful markers for embryo viability. Effects of exposures on uterine receptivity can also be quantified. Although the patients are themselves infertile, this clinical protocol allows us potentially to demonstrate effects of certain exposures, such as caffeine, on specific reproductive processes.