Work continued on both new methods development and ongoing hypothesis testing. The mechanism of action of phthalates was investigated, using animals of different ages to evaluate possible effects on the FSH receptor. Inhibition of the affinity of FSH binding was observed: the mechanism of this effect is currently under investigation. New methods were developed to evaluate testicular toxicity. Digital fluorescent imaging is being developed to assess early biochemical changes in toxicant-exposed cells. Significant progress was made in bringing this technique on-line. Two- dimensional gel electrophoresis methods were optimized to evaluate toxicant-induced changes in gene expression and post-translational modification. these methods are currently being applied to determine the mechanism of action of several Sertoli cell toxicants.