BACKGROUND: Leukocytes, both polymorphonuclear leukocytes(PMNL) and monocytes/macrophages have been implicated in the pathogenesis of the cerebral ischemia and stroke. Neutrophils accumulate in post-hypoxic-ischemic neonatal rat brain prior to the evolution of necrosis and neutrophil depletion attenuates hypoxic-ischemic brain injury. However, the mechanisms leading to post-hypoxic-ischemic neutrophil accumulation are unknown yet. We hypothesized that Intercellular Adhesion Molecule-1 might mediate post-hypoxic-ischemic injury in the immature brain; thus, we evaluated ICAM-1 gene expression in post-hypoxic-ischemic neonatal(postnatal day 7) rat brain. METHODS: Neonatal rats(n=36) underwent right carotid ligation followed by exposure in 8% O2 for 2.5 hours; this procedure typically produces ipsilateral striatal, hippocampal and cortical infarction. Control groups are included by carotid ligation alone, hypoxia alone, and neither hypoxia nor ligation. For RNA extraction, rats were killed 0, 1, 2, 4, 8, 12, 24 and 48 h post- hypoxia-ischemia and RT-PCR was carried out. RESULTS: ICAM-1 mRNA was barely detected in the controls including normal and sham operated animals. In the cortex, striatum and hippocampus, ICAM-1 mRNA was significantly induced in the ipsilateral(right) side compared to the contralateral(left) side just after hypoxia-ischemia. The elevated ICAM-1 mRNA gradually reached a peak at 4 or 8 h and then decreased to an almost basal level by 24 to 48 h. In contrast, the less pronounced contralateral(left-sided) ICAM-1 mRNA expression appeared to peak earlier, within 2 h post-hypoxia-ischemia. CONCLUSION: The temporal profiles of post-hypoxic-ischemic ICAM-1 mRNA expression are consistent with a role in post-hypoxic-ischemic neutrophil recruitment and in the evolution of subsequent brain injury.