cDNA clones complementary to liver mRNA were prepared and used to determine transcription rates of specific genes in isolated nuclei from liver, brain, and hepatoma cells. The cDNA sequences complementary to mRNA found only (or mainly) in the liver hybridize to labeled nuclear RNA only from liver nuclei. It appears that transcriptional events are primarily responsible for the synthesis of these, and perhaps most, tissue-specific moderately abundant mRNAs.