OBJECTIVE: To study the change of cleaved Caspase-3 expression within 24 hs following hypoxia ischemia brain damage (HIBD) in neonatal rats. METHODS: Seven day old rats were randomly assigned into control group and HIBD group. The Caspase-3 expression was assayed by the immunohistochemical staining and Western Blot analysis at 3, 6, 12 and 24 hs after hypoxia ischemia (HI). RESULTS: The positive staining of Caspase-3 transferred from cytoplasm to the nuclei and the cells were apparently condensed and shrunk within 24 hs after HI. The number of the cleaved Caspase-3 positive cells in the damaged cortex increased at 3 h, decreased at 6 h, increased again at 12 h and peaked at 24 h after HI, whereas the number in the damaged hippocampal CA1 region increased with time and peaked 24 hs after HI. The number of the Caspase-3 positive cell both in the cortex and the hippocampal CA1 region of the HIBD group at 3 h, 6 h, 12 h and 24 h was greater than that of the control group (P< 0.05 or 0.01 ). Caspase-3 expression was also found in the damaged brain tissue 3 hs after HI by Western blotting analysis. The intensity of the expression, decreasing at 6 h but increased again 12 hs after HI. CONCLUSIONS: The character of brain cell apoptosis within 24 hs following HIBD, increase of apoptotic cells twice, may be found with Caspase-3 as the marker for evaluating brain cell apoptosis in HIBD and it may be as reference for using anti apoptotic agents in treatment of HIE.