After synthesis and processing in the nucleus, mature transfer RNAs (tRNAs) are exported to the cytoplasm in a Ran.guanosine triphosphate-dependent manner. Export of defective or immature tRNAs is avoided by monitoring both structure and function of tRNAs in the nucleus, and only tRNAs with mature 5＇ and 3＇ ends are exported. ALL tRNAs examined can be aminoacylated in nuclei of Xenopus oocytes, thereby providing a possible mechanism for functional proofreading of newly made tRNAs. Inhibition of aminoacylation of a specific tRNA retards its appearance in the cytoplasm, indicating that nuclear aminoacylation promotes efficient export.