Effects of hyperbaric oxygen (HBO) on acute cerebral ischemia were studied in spontaneously hypertensive rats, which had the carotid artery bilaterally ligated. The animals were exposed to HBO (100% 02 at 2 ATA) for 30 min at 1 or 3 h after carotid ligation (treated group). Survival time and brain tissue metabolites were measured after HBO in these animals and compared with ischemic animals without HBO exposure (nontreated group). The animals treated at 3 h after ligation survived longer (6.5 +/- 0.7 h) than did nontreated ones (4.3 +/- 0.2 h) (P less than 0.05). The cerebral lactate increased much less in these treated animals (24.60 +/- 1.67 mM/kg) than in nontreated ones (31.78 +/- 1.68 mM/kg) (P less than 0.05). Cerebral ATP levels tended to decrease less in the former (0.66 +/- 0.17 mM/kg) than in the latter (0.59 +/- 0.07 mM/kg). When HBO started at 1 h after carotid ligation, however, there were no significant differences of survival time or brain metabolites between treated and nontreated groups of animals. The present results indicate that HBO administered at 3 h after brain ischemia prevents further increase in cerebral lactate and produces a slight but significant increase in survival time.