Effects of cordycepin on the incorporation of [³H] guanosine into embryonic Xenopus cells were examined. Cordycepin inhibited the labeling not only of poly(A)+RNA, but of all the the other major classes of RNAs. Cellular fractionation showed that this inhibition was much stronger in the labeling of cytoplasmic RNAs than of nuclear RNAs. [³H]Cordycepin was incorporated into both poly(A) +RNA and other RNA species.