Simultaneous expression of seven genes in Escherichia coli was measured by a reverse transcription-multiplex PCR fluorescence procedure. Genes studied were (i) oxyR(transcriptional regulator); (ii) katG, dps,gorA, and ahpCF (controlled by OxyR); (iii) sodA (controlled by SoxRS); and (iv) trxA(not related to OxyR or SoxRS). Except for trxA, transcription of all genes was activated during the course of growth of wild-type bacteria, though notable variations were observed with respect to both the time and extent of activation. WhereasoxyR, katG, dps, andgorA were activated during exponential growth,ahpCF and sodA were stimulated in stationary phase. Maximal induction ranged from 4.6- to 86.5-fold, forgorA and dps, respectively. Treatment with H₂O₂ stimulated expression of the genes (katG, dps, ahpCF, andgorA) previously identified as members of the OxyR regulon, except for oxyR itself. Induction by H₂O₂ was a remarkably rapid and reversible process that took place in an OxyR-dependent and ς^S-independent manner. NaCl induced expression of the genes controlled by OxyR, including the oxyR locus. This transcriptional up-regulation was preserved in a strain with the ΔoxyR::kan mutation, but it was abolished (ahpCF) or significantly reduced (oxyR and dps) in a strain with therpoS::Tn10 mutation, potentially reflecting positive transcriptional regulation of the oxyRregulon by ς^S. Expression of trxA was not increased either by H₂O₂ stress or by a shift to high-osmolarity conditions.