Uteroglobin, labelled with N-succinimidyl-(2-3-3H)-propionate, was applied in vivo for 3 h to pregnant rabbit uteri 7 and 9 days after mating. Light- and electron-microscopic autoradiographs showed that the endometrial epithelium, both ciliated and non-ciliated cells, is able to take up 3H-uteroglobin, however, with differing intensity. Large areas of labelling were found in the luminal epithelium, whereas the glandular epithelium contained fewer silver grains. Moreover, intensively labelled single cells or symplasms occurred in both luminal and glandular epithelium. They were identified as degenerating or dead cells. After internalization by pinocytosis or phagocytosis, the tritiated uteroglobin was observed in multivesicular bodies or in lysosomes with floccular content. Later, radioactivity was either found within residual bodies or distributed throughout the entire epithelium and the subepithelial stroma, i.e., the silver grains could no longer be assigned to specific cell organelles.