Chemiluminescence measurement in vivo system and its related studies on free radical reaction were reported. Weak chemiluminescence from rat brain surface subjected to ischemic cerebral stroke could be detected. It was observed just after embolization and lasted at least four hours. In order to investigate the mechanism of the generation of chemiluminescence and a side reaction of lipid peroxidation, several studies were made using 10% brain homogenate under oxygenated conditions at 37 degrees C. Brain homogenate showed increasing chemiluminescence that reached a plateau level a few hours later. Chemiluminescence, accompanying with thiobarbituric acid reactive substance (TBARS) formation, was based on free radical reaction with requirement of oxygen. Time course increase in TBARS formation, however, revealed some differences from that of integrated light intensity. Chemiluminescence spectrum in the visible region showed that excited indole chemicals (triplet states) generated in the present system emitted light during their return to ground state, but not singlet molecular oxygen. Suspension of acetone powder in Tris-HC1 buffer prepared from brain homogenate also disclosed chemiluminescence to some extent, whereas liposomes made of extracted brain lipid did not. These results probably indicate that proteins are essential for luminescence, but not lipid only, and involves the lipid-protein interaction in oxygenated brain homogenate. In the process of lipid-protein interaction, neither fatty acids composition nor protein molecules significantly altered, except for a newly-appeared high molecular weight protein. On the other hand, SH group was suggested to be quite vulnerable to free radical attack.(ABSTRACT TRUNCATED AT 250 WORDS)