The incorporation of bromodeoxyuridine (BUdR) into newborn rat tissue DNA has been determined after i.p. injection of 5-bromo-2'-deoxy-[6-3H]uridine. Incorporation of the unchanged nucleoside was shown by hydrolysis and ion exchange chromatography of extracted DNA. In all tissues examined, more than 90% of the radioactivity incorporated was in the form of bromodeoxyuridine.