Endometrial biopsy specimens were obtained from 46 normally cyclic heifers at known stages of their estrous period to show precise characteristic changes. These tissues were embedded in paraffin, sectioned, and stained with hematoxylin and eosin. The following histological changes were observed during the estrous cycle. Metrorrhagia was observed on Days 0 to 1 (estrus = Day 0). Mitoses in glandular epithelium occurred on Day 5. Basal vacuolation in the surface epithelium occurred on Days 5 to 6. Leukocyte invaded the functional layer on Day 7. Stromal mitoses were observed on Days 9 to 12. The results indicate that clincians need to be aware that histological evaluation is important for the diagnosis of endometrial function and that biopsy is useful for this purpose.