We have investigated the nature of the RNA that moves from cytoplasm to nucleus against a concentration gradient in Amoeba proteus. We find that: In the presence of actinomycin D an unlabeled nucleus grafted into a [³H]RNA cytoplasm acquires RNAs with sedimentation constants 30 S, 19 S, and 4-6S that are not related to the general population of cytoplasmic ribosomal and transfer RNAs. RNAs of sedimentation constants 39 S and 16 S may also enter the nucleus from the cytoplasm, but not in the presence of actinomycin D. Nuclei were transplanted from [³H]RNA cells through several unlabeled cytoplasms to dilute out migrating [³H]RNA. This resulted in the 4-6 S [³H]RNA being retained as the predominant labeled material of the nucleus and establishes that a substantial portion of 4-6S nuclear RNA does not leave the interphase nucleus. We conclude that nuclear RNAs may be classified in a new way: (1) RNA that is to become cytoplasmic RNA and presumably moves only from nucleus to cytoplasm; (2) RNA that migrates back and forth between nucleus and cytoplasm; and (3) RNA that does not leave the nucleus during interphase.