Transmission electron micrographs, with and without prior infusion of horseradish peroxidase, and scanning electron microscopy of fractured corpora lutea demonstrated extensive microvillus development on the surface of rat luteal cells 7 days after priming with PMSG and hCG. These microvilli were primarily on the capillary face of the luteal cells, while the basolateral faces of the cell were largely composed of smooth plasma membrane with some gap junctions between adjoining cells, Injection of a small dose of [125I]-hCG revealed localization of silver grains primarily on the microvillus surfaces and, to lesser extents, on the basolateral plasma membrane and in subplasma membrane cytosolic vesicles. Pinocytosis was clearly demonstrated. Injection of a dose of [125I]-hCG sufficient to desensitize the hCG stimulatable adenylate cyclase and to result in extensive loss of hCG receptors did not enhance the transfer of silver grains into cytoplasmic vesicles. This dose of 125I]-hCG decreased the area occupied by microvilli and increased the area of basolateral membrane.