Fish oil (FO) modifies cardiac membrane phospholipid fatty acid composition to confer increased efficiency of oxygen utilization and anti-arrhythmic effects. We tested the capacity of low dose increments of FO, rich in (n-3) polyunsaturated fatty acids ((n-3) PUFA), to reverse the detrimental pro-arrhythmic and inefficient oxygen usage effects of dietary saturated fat (including high ratio of (n-6) PUFA to (n-3) PUFA) during ischemia and reperfusion. Wistar rats were fed a saturated fat enriched (SAT) diet (15.3% fat, including 12% saturated fat, added by weight) for 6 wk and were then divided into four groups (n=10/group) fed that diet or a 12% fat diet containing 3,6,or 12% FO in place of SAT for 6 wk. Paced (300/min), erythrocyte-perfused isolated working hearts were subjected to low coronary flow ischemia (15 min) and were then reperfused. At normoxic baseline, external work capacity increased marginally at 6 and 12% FO; however, marked dose-related reductions in oxygen consumption were evident due to FO dependent reduction in oxygen-energy utilization efficiency, and associated reductions in coronary flow and oxygen extraction. Post-ischemic recovery resulted in lower oxygen consumption, greater oxygen-energy utilization efficiency, reduced coronary release of creatine kinase and reduced incidence of arrhythmias in all FO groups compared to the SAT group. Fish oil at a dose as low as 3% of total fat dietary supplement effectively reversed the high oxygen requirements and pro-arrhythmic effects of a saturated fat-rich diet, even with continued consumption of saturated fat (9%) in this ex vivo animal model.