The proteins of heterogeneous nuclear ribonucleoprotein (hnRNP) complexes are among the most abundant proteins in the nucleus. They bind nascent pre-mRNAs and remain associated with them through their nuclear processing into mRNA. Recent findings indicate roles for hnRNP proteins in the biogenesis of mRNA and reveal a surprising intracellular localization pathway for these proteins. Several of the hnRNP proteins shuttle continuously between the nucleus and the cytoplasm, and the reaccumulation of the exported hnRNP proteins in the nucleus occurs by a novel process that is dependent on transcription by RNA polymerase II. These findings suggest possible novel functions for hnRNP proteins in the cytoplasm and in the nucleocytoplasmic transport of mRNA.