We have shown that vitamin A (VA) and retinoic acid (RA) synergistically increase lung retinyl ester content in neonatal rats. To confirm whether this biochemical synergism attenuates early neonatal hyperoxic lung injury in mice, we exposed newborn C57BL/6 mice to 95% O2 or air from birth to 4 d. The agent [vehicle, VA, RA, or the combination vitamin A+retinoic acid (VARA)] was given orally daily. Lung and liver retinyl ester content was measured, and lung injury and development were evaluated. We observed that lung, but not liver, retinyl ester levels were increased more by VARA than by VA or RA alone. Hyperoxic lung injury was reduced by VA and RA, and more so by VARA. VARA attenuated the hyperoxia-induced increases in macrophage inflammatory protein (MIP)-2 mRNA and protein expression, but did not alter hyperoxia-induced effects on peptide growth factors (PDGF, VEGF, and TGF-beta1). The 4-d exposure to hyperoxia or retinoids did not lead to observable differences in lung development. We conclude that the VARA combination has synergistic effects on lung retinyl ester concentrations and on the attenuation of hyperoxia-induced lung injury in newborn mice, possibly by modulation of inflammatory mediators.