We constructed cloned DNA sequences complementary to unselected mRNAs [poly(A)+ cytoplasmic RNA] from Chinese hamster ovary cells and used them in RNA:DNA hybridization experiments. Each cloned DNA hybridized a single mRNA from 1.3-3.5 kb in length. The relative rates of labeling (transcription rates) of nuclear RNA complementary to each individual DNA segment varied approximately 10 fold. The relative cytoplasmic concentration of the same specific RNA sequences in the mRNA after an equilibrium labeling of the cells varied approximately 100 fold. In addition, we estimated the sizes of the nuclear RNA precursor molecules to these cytoplasmic mRNAs. Four main conclusions arise from these studies. First, the primary RNA transcripts, which range in size from 2.4-13.5 kb, are 2-6 times larger than the mRNAs; second, each cloned DNA segment is complementary to only one species of mRNA; third, for the RNA complementary to at least three of the nine cloned DNA segments, the relative cytoplasmic content is considerably different from the relative rate of nuclear RNA synthesis, suggesting the post-transcriptional events are involved in the determination of the cytoplasmic concentrations of some mammalian mRNAs; and fourth, the fraction of total nonribosomal nuclear RNA complementary to the nine cloned DNA segments is in most cases 10 fold less than the fraction of cytoplasmic mRNA complementary to the same cloned DNA segments, suggesting the synthesis of many hnRNA molecules that are qualitatively different from those which eventually contribute mRNA to the cytoplasm.