Pulse-chase experiments with [3H]-uridine were performed with glucosamine-treated chick cells infected by RNA viruses. It was established that in cells infected by fowl plague virus both virus RNA and the RNA with the complementary base sequence were metabolically stable. Newcastle disease virus-specific RNA was also metabolically stable in chick fibroblasts. In cells infected by Semliki Forest virus, radioactivity was not lost from the virus-induced 26 S RNA, while the activity of the partially RNase-resistant 20 S RNA could be chased to RNase-sensitive RNA of high mol. wt.