We previously reported that intraventricular administration of MEK1 inhibitor PD98059 decreases infarct volume after 2 hr of middle cerebral artery occlusion (MCAO) in mice (Proc Natl Acad Sci USA 96:12866–12869, 1999). Moreover, we found that delayed intravenous systemic administration of another MEK inhibitor U0126 during ischemia reduces infarct volume resulting from 3 hr of MCAO in mice (SFN Abstract Part1:286.13, 2000). In the present study, we investigated the role of MEK/ERK pathway in neuronal death induced by 9 hr of oxygen deprivation (OD) by which 70–80% of neurons die at 24 hr after reoxygenation (RO). Nine hour of OD reduced phosphorylation level of ERK2. Two hour of RO following 9 hr of OD increased phosphorylated ERK1/2. Incubation with U0126 inhibited phosphorylation of ERK2 at 0–3 hr after RO in a dose-dependent manner, which was correlated with neuroprotection. The neuroprotection by U0126 was dose-dependent, and maximum protection was achieved at 10 µM. However, U0126 did not reduce the ...