The study was undertaken to investigate the structure of the Leydig cells of the vervet monkey, Chlorocebus aethiops, using both light and electron microscopic techniques and to elucidate the association of these cells with other testicular cells. The Leydig cells occur as a group of polymorphic cells in the intertubular region, and are usually associated with large and small blood vessels and abundant loose connective tissue. Leydig cells have large spherical or slightly elongated nuclei, which possess prominent nuclei and frequently deep indentations of nuclear envelope. The nuclei are characterized by patches of euchromatin and heterochromatin regions with the latter usually occupying the periphery of the organelle. The cytoplasm contains numerous mitochondria containing distinct lamellar cristae, small patches of rough endoplasmic reticulum as short cisternae, a large dense network of interconnecting tubular smooth endoplasmic reticulum, and round or oval lysosomes. Abundant lipid inclusions and collagen fibers are also found in the cytoplasm of these cells. These cells are found associated with the spermatogonia type B, pachytene spermatocytes and Sertoli cells. The results from this study show that there are basic similarities in the Leydig cells of the vervet compared to other mammals, humans and nonhuman primates, however, there also seem to be species differences among the mammalian group. The association of these cells with developing cells, and Sertoli cells was established. However, the association of Leydig cells with spermatogenic cells at different stages of spermatogenesis needs further investigation.