The effect of vitamin E, betamethasone and mannitol upon a series of pathological free radical reaction within the hypoxic brain tissue was evaluated by chemiluminescence method. The hypoxic brain was induced by arterial hypoxemia (PaO2 17-22 mmHg) with normocapnia (PaCO2 28-38 mm Hg) and normotension (MABP 100-140 mmHg). 4% O2-96% N2 mixed gas was used as the replacement for obtaining lowered PaO2. In the control group high valued chemiluminescence was measured in the hypoxic state and in the early stage of the initial post-hypoxic state. In the groups administered vitamin E, betamethasone, mannitol and combination of them, however, just extra low valued chemiluminescence was detected. Besides to explore the stage on which the drugs act in lipid peroxidation, chemiluminescence spectra was analyzed using the homogenate added the each drug. Intensity peaks of the spectra were around at 480, 520-530, 570, 620-640, 680-700 nm before addition of the drugs. All the intensity peaks diminished after addition of vitamin E and betamethasone, while in case of mannitol, very little decrease of the intensity peaks was revealed. These experimental results indicate as follows. The lowered chemiluminescence value may prove the possibility of vitamin E, betamethasone and mannitol as free radical scavengers or inhibitor of lipid peroxidation. Chemiluminescence spectro-analysis shows that vitamin E and betamethasone act on the break down of lipid hydroperoxide and mannitol act on hydroxy radical in lipid peroxidation.