Air-breathing organisms experience an elevated concentration of oxygen mainly under two conditions. One occurs at birth when the O2 tension in the lung increases from approximately 25 torr present in utero to approximately 100 torr. The lungs, in particular, are also exposed to hyperoxia when oxygen is administered for therapeutic reasons. Under hyperoxic conditions, increased lung antioxidant enzyme activity is important for survival. The molecular basis for the increase in antioxidant enzyme gene expression under these circumstances is not well understood; in hyperoxia-exposed neonatal rats the elevation of lung catalase activity is not due to an increased rate of transcription but is associated with an increased concentration of catalase mRNA due to enhanced stability of the mRNA (Clerch, L.B., Iqbal, J., and Massaro, D. (1991) Am. J. Physiol. 260, L428-L433). We now show that neonatal rat lung protein forms specific complexes with catalase mRNA; this binding is redox-sensitive since when oxidizing agents are added binding is abolished but is restored by reducing agents. Our data also indicate lungs from hyperoxia-exposed rats have a larger proportion of catalase RNA-binding protein in oxidized form than lungs from air-breathing rats. This redox-sensitive binding of protein to catalase mRNA may be important in the control of catalase gene expression.