The snRNAs are abundant and stable components of t h e interphase nucleus. Aqueous and non-aqueous cell fractionation demonstrate that t h e snRNAs appear transiently in the cytoplasm shortly after transcription, before returning permanently to the interphase nucleus. In pulse label and chase experiments, the newly synthesized snRNA species appear in the cytoplasm after 1 min of labeling and then return to t h e interphase nucleus after approximately 15 min in the cytoplasm. In order to study t h e maturation and intracellular transport of these particles, a battery of metabolic inhibitors and alterations in cell culture conditions were investigated for their ability to interfere with t h e return of the newly synthesized snRNAs to the nucleus. A wide range of inhibitors of the cytoskeleton did not interfere with this process. Only the inhibition of protein synthesis and exposure of cells to medium of at least twice the normal tonicity block the return of the snRNAs to the nucleus. lmmunofluorescent staining of cells exposed to hypertonic medium identifies discrete foci in the cytoplasm that stain with the Sm antiserum, directed against proteins associated with t h e snRNAs. Using a detergent extraction procedure that preserves t h e cytoskeleton, t h e newly synthesized snRNAs in t h e cytoplasm fractionate as soluble complexes. These data are consistent with the hypothesis that the snRNAs partition into the interphase nucleus because of a preferential solubility and the existence of specific binding sites.