Electron microscopy evaluation indicated that the large luteal cells from control (saline-injected) cows were morphologically similar: 42-72% of the large luteal cells, in which the nucleus was present in the tissue section, contained secretory granules (100-300 nm diameter). From cell to cell, clustering of granules was observed in various regions of the cell cytoplasm, ranging from locations next to the nucleus to those next to the cell membrane where exocytosis was noted. The large mitochondria containing dense inclusions (500-1800 nm diameter) were uniformly distributed throughout the cytoplasm of 98% of the large luteal cells. The morphology of large luteal cells at 2.5 and 5 min after PGF-2 alpha treatment (25 mg) was similar to that of saline controls. However, at 15 and 30 min after PGF-2 alpha the secretory granules were observed primarily at the cell periphery and at 15 min only 22% of the large luteal cells contained secretory granules. Exocytosis involving fusion of the granule and cell membrane was more evident at this time. At 60 min after PGF-2 alpha few large luteal cells (0.5%) were observed with small granules. We suggest that the secretory granules represent those organelles responsible for packaging secretory proteins for transport out of the cell, and PGF-2 alpha is an effector that will initiate cell depletion of these granules and their contents.