The messenger RNA (mRNA) coding for uteroglobin has been localized in the rabbit uterus and lung by in situ hybridization. Tissue sections fixed in ethanol-acetic acid were hybridized to the cloned complementary DNA probe labeled with tritium. The hybridization sites were detected by radioautography. Control experiments using [3H]pBR322 DNA demonstrated the specificity of the observed labeling. In the lung, uteroglobin mRNA, present in small concentrations, could be clearly visualized only after background was decreased by incubation of sections with S1 nuclease. In pregnant rabbit uterine horns, uteroglobin mRNA, visualized by silver grains, was found in the endometrial epithelium. The concentration was greater in the cells of glandular epithelium than in the cells of surface epithelium. Specific and intense labeling was spread through the cytoplasm. Practically all epithelial cells contained uteroglobin mRNA. Hybridization was very weak in the uterine epithelial cells of the nonpregnant rabbit. In the lung, a high degree of labeling occurred on the ciliated and bronchiolar cells of the epithelium of bronchi and bronchioles whereas the goblet cells remained unlabeled. Certain cells lining alveolar ducts and alveoli in the pulmonary parenchyma also showed a slight labeling. No differences in the labeling were observed in the lung of either pregnant or non-pregnant animals. There are several differences in the intensity and distribution of labeling between our hybridization experiments and previous studies involving immunocytochemical detection of uteroglobin protein. The latter technique thus probably not only reflects the pattern of synthesis of the protein but also depends on uteroglobin retention in the cells. Moreover, no evidence was found to bear out the hypothesis that some endometrial cells which contain uteroglobin do not synthesize this protein but take it up from endometrial fluid.