The interstitial tissue of the mouse testis contains, besides mature and immature Leydig cells, fibroblasts, occasional plasma cells and a considerable number of macrophages abutting on the Leydig cells. Mature Leydig cells extend filopodia into dilated portions of the intercellular space. Along the cell surface deep hollows are frequently formed which contain numerous filopodia protruded from the plasma membrane limiting the hollow. This unique structure is tentatively designated the "basal infolding-like structure." Abundant SER is classified into the tubular and vesicular forms. The former is composed of interconnected tubules which may form flattened, partially fenestrated cisternae in some areas. The latter form is composed of vesicles of variable sizes. The tubules probably transform into the vesicular form, possibly reflecting the functional phases of Leydig cells. Thus, among vesicles, patches of tubules with smaller calibers may be detectable. Membranous whorls composed of concentric lamellar stacks of flattened, partially fenestrated cisternae of the SER are frequently observed, enclosing a mitochondrion, lipid droplet or microbody in their center. A typical annulate lamellae are occasionally found in the cytoplasm or in the whorl which is continuous to the membranes of the ER. Small foci of the RER are detected and sometimes dilations of its cisternae are observed. Polysomes are distributed not only in the RER foci, but also throughout the cytoplasm intervening among the SER membranes. Mitochondria possess tubular cristae and occasionally contain sparse ribosome-like granules. Out of several Golgi complexes dispersed in the cytoplasm, the juxtanuclear one contains a diplosome and an occasional solitary cilium. Relatively numerous microbodies are scattered in the cytoplasm. Lipid droplet content varied strongly from cell to cell, probably suggesting variable intensities of steroid synthesis. In the peripheral cytoplasm, large vacuoles were found, probably derived from the testosterone-containing SER. These possible secretory vacuoles occurred close to the plasma membrane, suggesting emiocytotic release of the contents. The basal lamina was restricted to certain areas of the plasma membrane.