C-type virus particles were found on electron-microscopic examination in placentas from two out of four young healthy Sprague-Dawley rats. One of these specimens contained virus particles budding from the plasma membranes of cells in the junctional zone of the placenta, i.e., the region where the fetal and maternal cell layers meet. In the other placenta, immature and mature C-type virus particles were found among cell debris also in the junctional region. This observation adds another species of animals to those recently reported, such as rhesus and baboon monkeys, as well as humans, in which C-type virus particles were found in the placenta. The presence of C-type viicant in view of the fact that a considerable number of these animals develop spontaneously a variety of malignant tumors, occasionally also leukemia and malignant lymphomas; however, none of these spontaneous tumors reveals the presence of virus particles on electron-microscopic examination. The nature of virus particles detected in rat placenta remains to be determined. As a working hypothesis, it is possible to assume that they may represent the passage of latent, presumably oncogenic, viruses transmitted "vertically" from parents to offspring. In the course of this passage some of them may be formed, emerging temporarily from their latency, before losing their identity and being again incorporated into the cell genetic components.