Neonatal rodents are more tolerant to hyperoxia than adults. We determined whether maturational differences in lung NF-kappaB activation could account for the differences. After hyperoxic exposure (O2 > 95%), neonatal (<12 hours old) lung NF-kappaB binding was increased and reached a maximum between 8 and 16 hours, whereas in adults no changes were observed. Additionally, neonatal NF-kappaB/luciferase transgenic mice (incorporating 2 NF-kappaB consensus sequences driving luciferase gene expression) demonstrated enhanced in vivo NF-kappaB activation after hyperoxia in real time. In the lungs of neonates, there was a propensity toward NF-kappaB activation as evidenced by increased lung I-kappaB kinase protein levels, I-kappaBalpha phosphorylation, beta-transducin repeat-containing protein levels, and total I-kappaBalpha degradation. Increased lung p-JNK immunoreactive protein was observed only in the adult lung. Inhibition of pI-kappaBalpha by BAY 11-7085 resulted in decreased Bcl-2 protein levels in neonatal lung homogenates and decreased cell viability in lung primary cultures after hyperoxic exposure. Furthermore, neonatal p50-null mutant (p50(-/-)) mice showed increased lung DNA degradation and decreased survival in hyperoxia compared with WT mice. These data demonstrate that there are maturational differences in lung NF-kappaB activation and that enhanced NF-kappaB may serve to protect the neonatal lung from acute hyperoxic injury via inhibition of apoptosis.