We evaluated the effect of hyperbaric oxygen (HBO) therapy on neurological function and infarct size in 33 unanesthetized cats subjected to temporary 6-hour or 24-hour occlusion of the middle cerebral artery (MCA) 7 to 10 days after transorbital implantation of a vessel occluder. HBO therapy (100% oxygen at 1.5 atmospheres absolute) was administered for 40 minutes during or after 6-hour occlusions and before, during, and after 24-hour occlusions. Neurological function was graded on a scale of 0 to 10 every 30 minutes before, during, and after occlusion and HBO treatments until it stabilized and then daily until the cats were killed 10 days after occlusion. The results were compared with observations in 13 untreated controls and 6 cats that received 100% O2 at atmospheric pressure during a 6-hour MCA occlusion. HBO therapy during the 1st or 3rd hour of a 6-hour MCA occlusion resulted in a four-grade improvement of the initial neurological function; this effect persisted during the remainder of the occlusion. The average grade of neurological deficit at death was 94% less than in the untreated cats (P less than 0.03). Infarct size in the HBO-treated group was 58% less than in controls (P less than 0.03). There was no significant difference in infarct size between the untreated cats and those treated with 100% O2 at atmospheric pressure. HBO therapy during the 4th hour of a 6-hour MCA occlusion had no statistically significant effect on infarct size, even though the mean neurological deficit was 73% less than in controls (P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)