A key to understanding the relationship between steroidogenesis and spermatogenesis is to elucidate qualitatively and quantitatively the steroidal micro-environment associated with specific germ cell stages and to determine the cell types responsible for steroid production and the intratesticular sites of steroid action. Such studies would also address the mechanism by which interstitial functions are coordinated with events in the tubules. The organization of the testis of common laboratory rodents and man is such that three or more generations of germ cells are intermixed in any one tubular cross-section. To circumvent this problem, we have chosen to study animal species in which (1) germ cells in different stages of maturation are segregated topographically within the testis; (2) Leydig cells or specific germ cell/Sertoli cell associations may be obtained in high yield in isolation from other testicular elements; (3) normal morphological and physiological changes in the testis occur during seasonal reproductive cycles. Using light and electron microscopy, steroid tracer methodology, steroid radioimmunoassays, estrogen receptor measurements, and cell culture techniques, we will (1) compare the androgena and estrogen synthesizing capabilities of the interstitial and tubular compartments of the testis of rodents (squirrel, guinea pig, chinchilla, mouse) and other vertebrates (human fetus, stallion, boar, Cnemicophorus, Necturus, Squalus, Notophthalmus/Taricha) in order to identify features common to all and differences that may correlate with Sertoli or Leydig morphology; (2) study structure and steroidogenic potential of testicular constituents during the annual cycle in seasonal breeders (squirrel, bat, Chrysemys, Necturus, Squalus, Notophthalmus/Taricha) and to further correlate steroid production with germ cell maturation i species in which stages are topographically segregated (Necturus, Squalus, Notophthalmus/Taricha); (3) attempt to locate cytosolic estrogen receptor in interstitial vs. tubular compartments and to define change which may be associated with seasonal cycles (squirrel, guinea pig, mouse chinchilla, Necturus, Cnemidophorus, stallion, boar, human fetus). Preliminary studies show the feasibility of using animal models outside the range of common laboratory species to materially advance our understanding of testicular function.