Sertoli cells play a key role in the spermatogenic function of the testis. Progress has been made in understanding the mechanism of action of follicle-stimulating hormone (FSH) on Sertoli cells from prepubertal and adult rats. In the past 2 years, research has focused on a number of components of the FSH-mediated signal transduction pathway, eg, G-protein synthesis, types of protein kinases activated, and regulation of cyclic AMP-response element-binding protein. In addition, the cloning of the complementary DNA and gene for the FSH receptor have led to greater insights into the structural basis of the FSH response. A continuing controversy over the necessity for FSH action in the adult rat testis led to studies that strongly suggest that androgens are sufficient to maintain quantitatively and qualitatively normal spermatogenesis. The paracrine interactions in the testis are another research area where major advancements have been made. The origin and actions of transforming growth factor-α and β and the role of activins and inhibits in testicular cell development have been recent subjects of investigation.