OBJECTIVE: To study the effect of MK-801, the antagonist of NMDA receptor, on Caspase-3 activation and apoptosis after cerebral hypoxic ischemic injury in neonatal rats. METHODS: Sevendayold rat pups were injected with either 0.5 mg/kg MK-801 or normal saline immediately after cerebral hypoxicischemic injury (HI). The pups were killed 24 h after the injection. Brain damage was evaluated using MAP-2 immunostaining. Caspase-3 activation was examined using antip17 subunit antibody. Apoptosis was examined by in situ labeling of hairpin probe (HPP). The infarction area was calculated according to MAP-2 staining. Active Caspase-3 cells and HPP positive cells were counted in the MAP-2 negative area of the cortex. RESULTS: The infarction area was reduced from (52±12)% to (23±5)% after the treatment with MK-801. A few Caspase-3 and HPP positive cells were also found in the cortex of normal rat brains. The number of positive cells reached a peak at 24 h after HI. Both the number of Caspase-3 (from 65±8 to 40±6.7 per high power visual field) and HPP (from 61.6±11.5 to 12.6±5.2 per high power visual field) positive cells were decreased after the treatment with MK-801. CONCLUSIONS: MK-801 inhibits Caspase-3 activation and reduces the number of apoptotic cells. NMDA is involved in the activation of the apoptotic process in the immature brain after HI.