Kinetics of DNA synthesis was estimated by autoradiography in the organ cultures of murine embryonic lung tissue. Strain A mice were taken for this experiment (intact and exposed to transplacental action of urethane--30 mg/g to a pregnant mouse, subcutaneously, once). The explants were examined 1,7, 15 and 21 days after the cultivation. Transplacental urethane inhibited DNA synthesis the first 24 hours. The label index became approximately 3 times less. The next two days it was restored and then stimulated. The maximal label index was noted in the experimental explants on the 7th cultivation day, i.e. on the 8th-10th day after the transplacental action of urethane on the lung tissue in utero. By the 15th cultivation day the label index dropped but was higher than in the intact explants; by the 21st day DNA synthesis in the intact cultures ceased completely, whereas in the experimental animals--it continued.