Cells generally adapt to long-term hypertonic stress by accumulating organic osmolytes. PAP-HT25 renal medullary cells in hypertonic medium accumulate sorbitol through a reaction catalyzed by aldose reductase and betaine through osmotically regulated transport. Hypertonicity increases aldose reductase protein synthesis rate by elevating its mRNA abundance. To test whether the rise in aldose reductase mRNA is due to enhanced transcription, PAP-HT25 cells adapted to isotonic medium were switched to hypertonic medium, and transcription rate was measured by nuclear run-on. Aldose reductase transcription rate peaked at 17-fold the isotonic level after 12 h of hypertonicity. Then, transcription fell as sorbitol and betaine accumulated. Transcription stabilized at fivefold the isotonic level within days. Aldose reductase mRNA stability was not significantly different between the hypertonic and isotonic steady states. Thus aldose reductase mRNA is osmotically regulated through changes in its transcription. The osmotically induced rise in aldose reductase transcription is blunted by the accumulation of intracellular betaine and is exaggerated and prolonged by preventing the accumulation of both sorbitol (by aldose reductase inhibition) and betaine (by removal from the medium). This study presents the first description of osmoregulated gene transcription in animal cells.