In terms of their light microscopic appearance and fine structure the ovarian interstitial cells of the rabbit are typical steroid-secreting cells. They are characterized by an abundance of agranular endoplasmic reticulum, spherical mitochondria with closely packed lamellar cristae, lipid droplets which appear to arise independently of the endoplasmic reticulum, conspicuous Golgi areas, a cytoplasm containing ribosomes and variable numbers of glycogen granules. A feature of the differentiation of the cells from the theca interna of atretic follicles or the stroma is the enlargement of the multiple Golgi areas and the progressive accumulation of agranular endoplasmic reticulum, possibly by “budding” from the Golgi cisternae. “Light” and “dark” cells are observed, the latter being characterized by a more closely packed agranular endoplasmic reticulum which tends to be tubular in type, that of the “light” cell being vesicular. Electron dense material (lipid?) is found in the vesicles and tubules of the agranular endoplasmic reticulum and in the Golgi cisternae; it may indicate a role of these structures in the biosynthesis of steroidal hormone. No fine structural changes specifically associated with pregnancy were observed. Degenerative changes are common and are described. The role of the interstitial cells, especially in relation to the production of 20 alpha-hydroxyprogesterone, is discussed.