Neonatal rats have an increased tolerance to hyperoxia, which is associated with a diminished pulmonary inflammatory response compared to adults. To investigate this differing response, expression of the neutrophil adhesion molecules, L-selectin and CD18, and levels of soluble L-selectin were examined using flow cytometry and sandwich ELISA on air-exposed neonatal rat neutrophils at 0-24 and 72 hours and 7, 10, 14, and 21 days of age compared to the adult and after exposure to hyperoxia (> 98% O₂) for 56 hours in adults and for 72 hours and 7 days in neonates. Expression of L-selectin in 0-24-hour neonates was similar to adults, but was significantly lower than adults at 72 hours and 7 days (p=.011). Soluble L-selectin levels were significantly higher than those in adults in the 0-24 and 72-hour neonates (p<.001). CD18 expression in unstimulated and activated neutrophils of neonatal rats was higher at 0-24 hours than in the adult (p<.001), but thereafter, did not differ from adults. After hyperoxic exposure, L-selectin did not differ between the exposure groups but soluble L-selectin tended to increase in neonates after 7 days of O₂ exposure Finally, CD18 was significantly higher after hyperoxic exposure of the adult (p=.008), but did not change with oxygen exposure in the neonate. Based on these findings, we speculate that the differences between neonatal and adult rats in expression of L-selectin may contribute to delayed oxygen toxicity in neonatal rats.