Matrix metalloproteinases (MMPs) and chemokines seem to be induced by hyperoxia in preclinical studies. We hypothesized that O2 exposure immediately after birth is associated with altered blood spot MMP 9 and ? chemokine concentrations. The following analytes were measured on blood spots on d 1 and 3 of life, using luminex technology in 1059 infants (birth weights <1000 g) in the NICHD Neonatal Research Network: MMP 9, monocyte chemoattractant protein 1 (MCP 1), macrophage inflammatory proteins (1? and ?), and regulated upon activation, normal t cell expressed and secreted (RANTES). Infants administered O2 continually from 6 to 24 h of life (n = 729), when compared with those with <6 h exposure (n = 330), had significantly lower mean birth weight and higher rate of respiratory distress syndrome (p < 0.002). On d 3, MCP 1 was higher and RANTES lower among infants with early prolonged O2 exposure. After adjusting for covariates, prolonged early O2 exposure retained a statistically significant association with higher MCP 1 on d 3 (p = 0.003). The consistent association between O2 exposure and MCP 1 among extremely preterm infants suggests that further investigation of its role in oxidative injury is warranted.