Insulin improves myocardial contractile function during moderate ischemia, but the mechanism is unknown. To determine effects of insulin on myocardial oxygen utilization efficiency (O2UE) and energetics, regional left coronary perfusion pressure (CPP) was lowered sequentially from 100 to 60, 50, and 40 mmHg in 24 anesthetized, open-chest dogs. Regional power index (PI), myocardial oxygen consumption (MVO2), and O2UE index (PI/MVO2) were determined in untreated and insulin treated (4 U/min, i.v.) hearts. Biopsies were obtained from six untreated and six insulin-treated hearts at CPP=40 mmHg for determining high energy phosphates and the cytosolic phosphorylation potential. Measurements were compared with data from normal, untreated myocardium (n=11). MVO2 fell (P<0.05) in all hearts as CPP was lowered to 40 mmHg, and was unaffected by insulin treatment. PI decreased 32 and 75% in untreated hearts at CPP=50 and 40 mmHg, respectively (P<0.05). In insulin treated hearts, PI was not significantly depressed at CPP>40 mmHg, and fell only 26% at CPP=40 mmHg. O2UE increased (P<0.05) in all hearts at CPP=60 mmHg. In insulin treated hearts, O2UE was greater (P<0.05) at CPP=50 and 40 mmHg than at CPP=100 mmHg, and greater (P<0.05) than in untreated hearts at CPP=40 mmHg. Reducing CPP to 40 mmHg produced similar metabolic changes in all hearts. Compared to normal myocardium, ATP content of untreated and treated hearts was unchanged, creatine phosphate content decreased 21 and 14%, creatine content increased 24 and 30%, inorganic phosphate concentration increased 108 and 140%, and phosphorylation potential decreased 80 and 77%. We conclude that insulin markedly improves PI and O2UE without altering cytosolic energetics during moderate myocardial ischemia.