We explored the ability of increased oxygen pressure to modify necrosis in an open-chest rabbit model of myocardial ischemia and reperfusion. A branch of the left coronary artery was occluded for 30 minutes followed by 3 hours of reperfusion. Infarction was measured by triphenyl tetrazolium staining and expressed as a percentage of the ischemic zone. Untreated rabbits were ventilated with 100% oxygen at 1 atm absolute. Treatment animals were exposed to hyperbaric oxygen at 2.5 atm absolute. The 1.0-atm control hearts developed 41.5 +/- 4.6% infarction of the ischemic zone. Animals exposed to hyperbaric oxygen during ischemia only, reperfusion only, or ischemia and reperfusion had significantly smaller infarcts with respect to control animals (16.2 +/- 2.9%, 14.5 +/- 3.7%, and 9.8 +/- 2.7%, respectively; P < or = .01), indicating that they had been protected by the procedure. When hyperbaric oxygen was begun 30 minutes after the onset of reperfusion, no protection was seen (35.8 +/- 3.8%). We conclude that hyperbaric oxygen limits infarct size in the reperfused rabbit heart and that the effect can be achieved when hyperbaric oxygen is begun at reperfusion. Sterling, Thornton, Swafford, Gottlieb, Bishop, Stanley, Downey, , (1993). Hyperbaric oxygen limits infarct size in ischemic rabbit myocardium in vivo. Circulation, 1993 Oct;88(4 Pt 1):1931-6. https://www.ncbi.nlm.nih.gov/pubmed/8403338