The fine structure of boar Leydig cells is re-examined in samples post-fixed with OSO4-K4Fe(CN)6, an elective method to preserve the ultrastructural detail of the endocrine testis. The cells show a pleomorphic arrangement of the smooth endoplasmic reticulum. Large "vesicle-like" dilatations are interspersed in the tubular network of this membrane system. The dilatations contain a flocculent material with a dense core and a dispersed peripheral rim. Additional features of boar Leydig cells revealed by OSO4-K4Fe(CN)6 treatment are parallel tubular arrays within residual bodies, and electron-lucent inclusions suggestive of crystals of cholesterol or its esters. These structural characteristics were not identified in previous electron-microscopic studies on Leydig cells of the boar or of other mammals. The unique components of boar interstitial cells described may be the morphological counterpart of the peculiar composition of testicular steroids secreted in this animal, namely low concentrations of androgens and large amounts of pheromones.