The distribution of uteroglobin mRNA has been investigated in the endometrial compartments of the rabbit uterus during early pregnancy (day 0.5 p.c.--day 12 p.c.) using nonradioactive in situ hybridization. Digoxigenin-dUTP labeled oligodesoxy-nucleotide-probes (24mer) and an anti-digoxigenin-antibody conjugated with alkaline phosphatase were developed and used. It could be shown, that uteroglobin mRNA localization is restricted to the endometrial epithelium. There are differences in the extent of uteroglobin mRNA expression within the epithelial cells, which is in accord with our interpretation on the existence of different epithelial cell populations. From day 0.5 p.c. to day 9 p.c. the cells of the basal glands express uteroglobin mRNA continuously, whereas the proliferating surface epithelium shows a remarkable fluctuating pattern of uteroglobin mRNA expression. On day 2 p.c. the whole surface epithelium starts to express the uteroglobin message, and up to day 5 p.c. all these cells show a high level of uteroglobin mRNA expression, which drops significantly on day 6 p.c., when significant changes in the cyto-morphology of the surface epithelium for implantation occur. On day 7 p.c., there is no more uteroglobin mRNA expression in the surface epithelium, however remaining expression in the basal glands. The latter is evident up to day 9 p.c. From day 10 p.c. onwards, neither the luminal nor the deep glandular epithelium express any uteroglobin mRNA. Our observations on the cellular level have been continued in parallel studies on endometrial homogenates by Northern Blot analysis of uteroglobin mRNA (600 bases). Finally, it is discussed whether Uteroglobin mRNA may be an useful marker for the differentiation of various endometrial epithelial cell populations.