Recent studies suggest that peroxidation of arachidonic acid (AA) accumulating during ischemic insult, may be related to the occurrence of post-ischemic brain damage. Since the influence of the increased brain content of AA remains unclear, the present study was undertaken to explore whether the intracerebral injection of AA is associated with the injury of the surrounding brain tissues such as brain edema. Rats received the intracerebral injection of 10 microliters of test solution (160 microgram of AA emulsified in 1% bovine serum albumin, BSA) or the same volume of BSA as the control. The measurement of specific gravity (SG) (74 rats) and the local CBF by H2 clearance technique (21 rats), and electron microscopical studies with tracers (Evans blue and horseradish peroxidase, HRP) (24 rats) were carried out. The effect of pretreatment with indomethacin (PI, 10 mg/kg) was evaluated in separate groups. AA transiently produced a significant decrease in SG and in 1CBF adjacent to the injection sites (p less than 0.01). The increased pinocytotic transportation, an accumulation of edema fluid containing HRP in extracellular space and endothelial injuries were observed only after AA injection. The above-mentioned falls in SG and in 1CBF, and the extravasation of HRP were inhibited by PI. The fact that PI prevented the edema formation, 1CBF change and the disturbance of blood-brain barrier induced by AA, indicates that peoxidation of AA through the arachidonate cascade is involved in the mechanism of brain edema formation.(ABSTRACT TRUNCATED AT 250 WORDS)