Seven primiparous Japanese Black cows, which were nursing their calves, were slaughtered several days post partum to examine the histological pattern of uterine involution. At Days 18 and 23 post partum, the uterine endometrium contained many phagocytes and nodular aggregates of lymphocytes. The outer diameter of the uterine glands varied considerably and the lumen was enlarged. Capillaries located under the surface epithelium were not yet completely contracted. At Days 46 and 54 post partum, however, the endometrium contained only a small amount of lumphocytes and phagocytes and uterine glands evenly distributed showed a tall glandular epithelium. These glands which were located in the basal zone hardly appeared to have a lumen. Ultrasonographical observation of the progression of uterine involution was started at Day 8 post partum and continued until Day 43 post partum. The diameter and area of the uterine horn and the endometrium in cross-sectional images of the endometrium were estimated using an image analyzer. The relationship between the estimated dimensions of the uterine horn and the number of days post partum fitted into polynomial regressions. It was concluded that the uterine involution of cows with calves was completed at approximately Day 40 post partum.