Adult male Ellobius are partially deficient in testosterone synthesis, despite the fact that the interstitial tissue is well developed and apparently normal from observations using conventional light microscopy. The present study of the Ellobius Leydig cell by means of electron microscopy reveals structural peculiarities. An abundance of mitochondria of large size, the absence of liposomes and the numerous dense bodies (lipofuchsin deposits, material resulting from degenerating mitochondria, etc.) are the most striking features, which closely resemble those noted after gonadotropin stimulation in various mammalian species. The in vitro incubation of the mitochondrial fraction with [³H-] cholesterol demonstrates that the activity of the cholesterol side chain cleavage system is apparently higher than in the normal mouse under similar conditions. These abnormalities in the Ellobius testis confirm our previous hypothesis that LH level must be elevated in the blood of the adult male as a consequence of a testicular defect in testosterone synthesis, localized at the 17-ketoreductase level as demonstrated elsewhere.