Involution and regeneration of the endometrium after parturition in the ewe, were studied by light- and electron microscopy. The luminal epithelium in intercaruncular regions of the endometrium remained intact at all stages, but degeneration and death of many glandular epithelial cells were observed on the day after parturition. Glandular regeneration had commenced by 8 d post partum, and the glands were substantially regenerated by 15 d. Caruncular epithelial cells on the maternal side of the placentomes, between the bases of the maternal septa, persisted during the period of degeneration of the foetal and maternal tissues of the placentomes. Epithelial cells from this source contributed to the regeneration of the caruncular epithelium following shedding of plaques of degenerate placental tissue from the caruncles, which commenced after 8 d and was completed before 31 d. Thus, ingrowth of epithelium from the edges of the caruncles, as previously proposed, was not the sole source of new caruncular epithelium. The additional source of regenerating epithelium identified here may account for the rapidity with which epithelium appears in the centres of some caruncles, several millimetres in diameter, during endometrial regeneration. However, in some caruncles, regeneration of the epithelium was not completed until after 31 d post partum.