Systemic administration of salicylate (SA) to rats (100 mg kg-1 i.p. ) was used as an in vivo trap of hydroxyl radicals (.OH). In the brain SA reacts with hydroxyl radicals to form the stable adducts 2, 3- and 2,5 dihydroxybenzoic acid (DHBAs) which can thus be taken as an index of .OH formation. The DHBAs were recovered by intracerebral microdialysis in hippocampus or striatum and quantified by high pressure liquid chromatography (HPLC) with electrochemical detection. There were no peaks corresponding to 2,5-DHBA or 2,3-DHBA in the chromatograms from rats not receiving SA. A basal level of 2,5-DHBA was seen in the dialysates from all animals given SA whereas 2, 3-DHBA was not detected. In one group of rats generation of free oxygen radicals was induced in the striatum by adding Fe2+ and ascorbate to the perfusion fluid to test the sensitivity of the system. Addition of Fe2+ ascorbate to the perfusion fluid induced a significant 7-fold increase in 2,5-DHBA that gradually returned to baseline after removal of Fe2+/ascorbate. In two other groups the microdialysis probes were implanted in either the striatum or the hippocampus and the animals were subjected to 20 min of four-vessel occlusion + hypotension (4-VOH). Significant reductions in 2,5-DHBA were detected during ischaemia followed by significant increases of 5-fold and 3-fold in the striatum and hippocampus, respectively, beginning immediately upon reperfusion and lasting for the remainder of the observation period (160 min).