The early genetic response to hyperosmotic stress remains to be elucidated in eukaryotes. We observed that hyperosmotic NaCl in Madin-Darby canine kidney cells increased levels of mRNA encoding the immediate early gene (IEG) transcription factors Egr-1 and c-fos at 2 h of treatment by two- and threefold, respectively. Sham treatment and hyperosmotic glycerol, and ineffective osmole, had no effect. Hyperosmotic NaCl, but not glycerol, also increased the mRNA level of the stress protein HSP70 by four- to fivefold at 2, 6, and 24 h. These changes occurred despite inhibition of total RNA transcription rate and DNA synthesis rate by NaCl. Neither NaCl nor glycerol treatment manifested significant cytotoxicity. NaCl, and to a lesser extent glycerol, suppressed protein synthesis, a phenomenon previously correlated with IEG superinduction. Therefore, hyperosmotic stimuli with different physiological effects result in differential expression of IEGs and the stress protein HSP70; induction of the former may govern the ensuing program of gene expression that culminates in the osmolyte response, while the latter may serve as a temporizing protective measure.