This study was carried out to elucidate the pathophysiologic mechanism of cerebral hyperemia observed during the early phase of bacterial meningitis. We tested the hypothesis that microbial invasion through the blood-brain barrier is responsible for cerebral vasodilation and hyperemia in meningitis. Escherichia coli was given either intravenously (i.v.) or intracisternally (i.c.) to closely mimic the primary or secondary bacterial invasion occurring in meningitis and newborn piglets were grouped according to their invasion results (+ or -); 12 in the i.v. (+) group, 14 in the i.v. (-) group, 13 in the i.c. (+) group, 15 in the i.c. (-) group. The results were compared with eight animals in the control group. Near infrared spectroscopy (NIRS) was employed to monitor changes in total hemoglobin (HbT), oxygenated hemoglobin (HbO), deoxygenated hemoglobin (Hb), deduced hemoglobin (HbD), and oxidized cytochrome aa3 (Cyt aa3). HbT, as an index of cerebral blood volume, increased progressively in both i.v. (+) and i.v. (-) groups and became significantly different from control and baseline values at 2 h. Hb significantly increased only in i.v. (+) group. HbD, as an index of cerebral blood flow, decreased significantly in i.v. (+), i.v.(-) and i.c. (-) groups and this change was mitigated in i.c. (+) group, HbO was reduced in i.c. (-) group and this decrease was attenuated in i.c. (+) group. Increased Cyt aa3 was observed in all experimental groups after bacterial inoculation. Changes in ICP, blood pressure, cerebral perfusion pressure, blood or CSF glucose or lactate, CSF TNF-alpha level, or CSF leukocytes number were not associated with changes in NIRS findings. These findings suggest that primary or secondary bacterial invasion across the blood-brain barrier is primarily responsible for cerebral vasodilation and hyperemia observed during the early phase of bacterial meningitis.