The use of hyperbaric oxygenation (HBO) for the treatment of severe brain ischaemia remains controversial. The HBO may interfere with destructive neutrophil (PMN) infiltration following ischaemia/reperfusion. The effects of HBO on PMN accumulation and the area of ischaemic tissue damage were investigated in rats having permanent focal ischaemia (4 h). The right middle cerebral arteries of a group of Wistar rats were permanently occluded. The rats were then randomly divided into those ( n=7) to be treated with HBO at 2 atm for 230 min and those ( n=8) to breathe air at atmospheric pressure for an equivalent period. The HBO had no effect on permanent ischaemia, as there was no significant difference in the area of ischaemic tissue damage between HBO-treated [mean (SD)] [331 (88) mm(3)] and non-treated animals [322 (111) mm(3)]. Moreover, the increase in myeloperoxidase [5.4 (4.1) compared to 2.4 (1.2) pg x g(-1) wet weight of brain] was not significantly different. The results indicate that HBO did not reduce tissue damage during 4 h of permanent focal ischaemia.