The compartmentalization of the parenchyma of the corpus luteum in the dog was studied by both 100 and 1000 KV electron microscopy. The organells within the luteal cell are oriented with a high degree of consistency towards the pericapillary space. Characteristically, the avascular pole and the lateral margins of the cell posses predominantly stacked and whorled cisternae of agranular ER. In the central medial portions of the cell, pleomorphic mitochondria with tubulo-vesicular cristae and anastomosing tubules of agranular ER predominate. However, the distribution of organelles in this compartment is graded. Mitochondria predominate in the central medial areas while tubular ER is more dominant peripherally. Microfilaments are ubiquitous in this compartment and run a longitudinal course between and around the subcellular components towards the pericapillary space. The Golgi apparatus is large and prominent and is positioned over the pole of the nucleus that faces the basal lamina. Coated vesicles are abundant in the Golgi regions and along the lateral surface of the cell. Three distinct regional specializations of the cell surface exist. The basal surface contains long pleomorphic cytoplasmic folds that fill the pericapillary space, are interconnected by small gap junctions and contain abundant multivesicular bodies. The lateral cell surface is covered with microvilli and is organized into tortuous intercellular channels and canaliculi. These are interrupted at intervals by cytoplasmic protrusions that extend from one cell well into the cytoplasm of the next. Large, well-developed gap junctions line the margins of the cells furthest removed from the pericapillary space. Finally, the individual cells exhibit heterogeneity with respect to the amount one subcellular organelle or compartment is expressed relative to another. These observations are discussed in relation to the subcellular compartmentalization of progesterone synthesis and release.