PIP: Ovariosalpingectomized rat uterine glands and luminal epithelium were examined by electron microscopy and in serial cross sections under light microscopy after up to 8 days of treatment with 5 mg progesterone daily. Under light microscopy, the gland lumen was narrow or absent in many epon sections, but wide in many paraffin sections, filled with toluidine blue stained secretion, and serial sections showed that the openings were closed, allowing no connection between the gland lumen and the uterus. In electron micrographs, only those glands without an opening appeared altered by progesterone. The most notable differences in the glandular epithelium were microvilli, condensed ribosome-free cytoplasm next to the lumen, numerous vesicles, sacs and dilated Golgi cisternae in the apical cytoplasm, and more giant mitrochondria in the basal cytoplasm than usually seen in controls. In the luminal epithelium, there were 3 distinct regions: the apical region had condensed cytoplasm often extruded into the lumen, with close-packed, smooth, empty vesicles; the middle region had granular endoplasmic reticulum, mitrochondria, dense bodies, multivesicular bodies, and lipid granules; the basal region contained the nucleus, granular endoplasmic reticulum, mitrochondria and dense abodies. These observations were interpreted as indicative of a transitional state from secretion to absorption, especially since without an opening, secretion would be of little significance.