['Extracellular superoxide dismutase (Ec-SOD) may protect the heart against myocardial infarction (MI) because of its extended half-life and capacity to bind heparan sulfate proteoglycans on cellular surfaces. Accordingly, we used direct gene transfer to increase systemic levels of Ec-SOD and determined whether this gene therapy could protect against MI.', 'The cDNA for human Ec-SOD was incorporated into a replication-deficient adenovirus (Ad5/CMV/Ec-SOD). Injection of this virus produced a high level of Ec-SOD in the liver, which was redistributed to the heart and other organs by injection of heparin. Untreated rabbits (group I) underwent a 30-minute coronary occlusion and 3 days of reperfusion. For comparison, preconditioned rabbits (group II) underwent a sequence of six 4-minute-occlusion/4-minute-reperfusion cycles 24 hours before the 30-minute occlusion. Control-treated rabbits (group III) were injected intravenously with Ad5/CMV/nls-LacZ, and gene-therapy rabbits (group IV) were injected with Ad5/CMV/Ec-SOD 3 days before the 30-minute occlusion. Both groups treated with Ad5 received intravenous heparin 2 hours before the 30-minute occlusion. Infarct size (percent risk area) was similar in groups I (57+/-6%) and III (58+/-5%). Ec-SOD gene therapy markedly reduced infarct size to 25+/-4% (P<0.01, group IV versus group III), a protection comparable to that of the late phase of ischemic preconditioning (29+/-3%, P<0.01 group II versus group I).'] Does gene therapy with extracellular superoxide dismutase protect conscious rabbits against myocardial infarction?