Hypertonicity increases mRNA and protein abundance of the transcription factor TonEBP/OREBP, contributing to increased transcription of downstream osmoprotective genes. Previously, this was attributed to increased transcription of TonEBP/OREBP since no change was found in its mRNA stability. However, there is no direct evidence for increased transcription, and the 3'-UTR of TonEBP/OREBP contains numerous adenylate/uridylate-rich elements, which can modulate RNA stability. Therefore, we have reinvestigated the effect of hypertonicity on TonEBP/OREBP mRNA stability. We find that, in mIMCD3 cells, raising osmolality from 300 to 500 mosmol/kg by adding NaCl increases TonEBP/OREBP mRNA to a peak of 2.3 fold after 4 hours, followed by a decline. TonEBP/OREBP protein increases to a sustained peak of 3.0 fold at 8 hours. To determine the stability of TonEBP/OREBP mRNA, we measured the rate of its decrease after inhibiting transcription with actinomycin, finding that it is stabilized for 6 hours after adding NaCl. This stabilization is sufficient to explain the increase in mRNA without any change in transcription. To investigate how hypertonicity stabilizes TonEBP/OREBP mRNA, we tested luciferase reporters containing parts of the TonEBP/OREBP mRNA UTR. Inclusion of both the 5'- and 3'-UTR increases reporter activity, consistent with mRNA stabilization. Surprisingly, however, it is the 5'-UTR that stabilizes; the 3'-UTR, by itself, decreases reporter activity. Conclusions: 1) Hypertonicity stabilizes TonEBP/OREBP mRNA, contributing to its increase. 2) Stabilization depends on the presence of the 5'-UTR.