Eighteen pregnant cows were individually housed and allowed to calve normally. None of the cows retained fetal membranes. Tissues were obtained for study on Days 1, 3, 5, 7, 11, 15, 19, 23, 27, 31, 45 and 60 post partum. Because the exact time of calving of each animal was not known, the termDay 1' represented an approximated period of 24 hr beginning from the time of observation of the calf with its dam in the maternity stall. Following exposure of the internal genitalia as described by Schultz, Fahning & Graham (1966), caruncular and intercaruncular uterine samples were ob- tained from an area at the base of the uterine horn by an in-vivo serial surgical technique (Archbald, 1969). Uterine samples were also obtained by per- cervical biopsy (Skjerven, 1956) and at necropsy. All samples were fixed in 10 % phosphate-buffered formalin for approximately 5 days. After trimming, they were embedded in paraffin wax, and sections cut at 6 µ were stained with haematoxylin and eosin as described by Humason (1967). Table 1 shows the chronological sampling sequence for each cow, and Table 2 shows the total number of samples obtained on each day post partum and the method of sampling. The histological features of tissue sections from all the animals studied were similar. Sections of caruncular tissue revealed the presence of numerous remnants of chorioallantoic cells in maternal crypts on Day 1. These remnants underwent necrosis and mineralization, and were either phagocytized by macrophages observed in the area or expelled with the lochia. Neither necrotic nor mineralized chorioallantoic cells were observed in the caruncle after Day 11 post partum. Gier & Marion (1968) reported the presence of chorioallantoic cells in the lumen of maternal caruncular crypts at Day 2 post partum. It therefore appears that detachment of terminal portions of chorioallantoic villi in mater¬ nal crypts and shrinking of chorioallantoic villi and dilatation of maternal caruncular crypts (Roberts, 1956) probably play a major role in separation of fetal and maternal placentae. Progressive degenerative vascular changes were observed in the medium- sized and smaller arteries of the caruncular stratum compactum from Days 1 to 19. These changes consisted of hydropic degeneration of the cytoplasm and pycnosis of the smooth muscle cells of the tunica media and fibrinoid necrosis of