Dietary enrichment with fish oil-derived n-3 polyunsaturated fatty acids has been shown to suppress the arrhythmias that occur during postischemic reperfusion. We have recently implicated a rapid release of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] during postischemic reperfusion in the generation of these arrhythmias. The effects of dietary supplementation with fish oil on both cardiac Ins(1,4,5)P3 and arrhythmogenic responses to reperfusion were therefore investigated in perfused rat hearts. Comparisons were made with control and n-6 polyunsaturated or saturated fat-supplemented diets. In control hearts, reperfusion increased Ins(1,4,5)P3 levels [from 9 +/- 2 at 20 min ischemia to 26 +/- 3 counts per minute (cpm)/mg protein with 2 min of reperfusion] and produced a high incidence of ventricular tachycardia (92% VT) and ventricular fibrillation (85% VF). Dietary fish oil supplementation, which increased composition of n-3 fatty acids in myocardial membrane phospholipids, prevented the reperfusion-induced rise in Ins(1,4,5)P3 (11 +/- 1 at 20 min ischemia and 12 +/- 2 cpm/mg protein after 2-min reperfusion) and significantly suppressed reperfusion arrhythmias (38% VT, 13% VF; P < 0.01 vs. control group). Thus the inhibition of reperfusion-induced rises in Ins(1,4,5)P3 by n-3 polyunsaturated fatty acids after dietary fish oil supplementation provides a possible mechanism for the inhibitory effect of n-3 fatty acids on reperfusion-induced arrhythmias.