The expression of a number of different mammalian genes is directly affected by hypertonicity. At present, the list of their products includes aldose reductase, heat shock proteins, early response factors, and transporters for betaine, inositol, and taurine. Hypertonicity increases the abundance of the mRNAs for all of them. Aldose reductase mRNA levels increase because of increased transcription with little change in the stability of its mRNA. Transcription of the betaine transporter also increases. The mechanisms by which hypertonicity increases the transcription of these mammalian genes remain speculative. However, the consideration of transcriptional control of betaine transport in bacteria and of heat shock proteins in many organisms provides interesting insight into this question.