Monocyte chemoattractant protein-1 (MCP-1) regulates monocyte accumulation in several macrophage-dependent experimental disease models. In the neonatal brain, activated microglia accumulate rapidly after hypoxic- ischemic injury. These cells produce potentially neurotoxic factors that may contribute to the progression of injury. To determine whether MCP-1 could be one of the molecular signals that influences the microglial response to hypoxic-ischemic injury in the neonatal brain, we examined the impact of acute hypoxic-ischemic injury on MCP-1 mRNA and protein expression. Seven- day-old rats underwent right carotid artery ligation, followed by 3 hours of 8% oxygen exposure, to elicit ipsilateral forebrain hypoxic-ischemic injury. To detect MCP-1 mRNA in situ hybridization assays were performed using 35S- labeled antisense riboprobes generated from rat MCP-1 cDNA. Animals were evaluated 0, 1, 2, 4, 8, 16, 24, 48, and 120 hours after hypoxic exposure (N ≤ 3/group). Immunocytochemistry (with a polyclonal rabbit antirat MCP-1 antibody) was used to determine the anatomic and temporal distribution of MCP-1, in samples obtained 10 minutes to 5 days after hypoxic exposure (N ≤ 3/group). Monocyte chemoattractant protein-1 mRNA was first detected in periventricular regions of the lesioned hemisphere 1 hour after hypoxia- ischemia; periependymal and intraparenchymal MCP-1 mRNA expression were detected at 4 hours; hybridization signal peaked at 8 to 24 hours; and no MCP-1 mRNA was detected at 48 and 120 hours. In lesioned forebrain, MCP-1 protein expression were consistently detected at 2.5 to 48 hours after hypoxia-ischemia. Many immunoreactive cells appeared to be neurons. These results suggest that in the developing brain, MCP-1 could represent a functionally important molecular signal for the microglial response to hypoxic-ischemic injury.