AbstractFine structural observations on the corpus luteum of late pregnancy in rhesus monkeys (Macaca mulatta) are described. Ovariectomy was performed between 141 and 146 days of pregnancy. Granulosa lutein cells measured 25 to 30 μ and were supplied by fenestrated capillaries. The plasma membranes of neighboring cells formed intercellular canaliculi which were bordered by microvilli. The nuclei were large and contained a nucleolus and peripheral heterochromatin. The central region of the cytoplasm contained a high concentration of: (1) rod‐shaped mitochondria with tubular cristae and some electron‐opaque inclusions in their matrices, (2) elaborate Golgi complexes and associated large tubular cisternae, and (3) tubular as well as cisternal agranular endoplasmic reticulum. The more peripheral region of the cytoplasm was filled with: (1) well developed parallel arrays of granular endoplasmic reticulum, (2) bundles of 50 Å filaments with dense areas, and (3) dispersed lipid droplets. Membrane‐bound granules of various types were observed in most cells. Large lacunae were present in the cytoplasm which were not confluent with the intercellular canaliculi or the perivascular spaces. Microvilli projected into the empty‐appearing lumen of the lacunae. It was concluded that the morphological features of the granulosa lutein cells of late pregnancy were consistent with those considered to be necessary for steroidogenic