Ischemic preconditioning (IPC) in the heart may reduce myocardial energy demand. The present study was undertaken to examine changes in myocardial oxygen consumption (MVO2) during ischemia by IPC in Langendorff perfused rat hearts. We assessed MVO2 during ischemia from the measurement of mitochondrial cyt. aa3 redox state by a two-wavelength reflectance spectrophotometry where T(1/2), the time from the onset of ischemia to the point for half reduction of cyt. aa3, was assumed to represent MVO2. The heart was preconditioned by three cycles of 5 min ischemia plus 5 min reperfusion and then subjected to 30 min global ischemia followed by reperfusion for 30 min. The T(1/2) was significantly longer in the preconditioned heart (30 +/- 6 s, n = 10) than the control heart (14 +/- 5 s, n = 9, P<0.001), indicating a reduction of MVO2 during ischemic period by IPC. The prolongation of T(1/2) was evident after only one IPC episode. When the heart was perfused with high K+ solution to abolish MVO2 for contractions, we still found the prolongation of T1(1/2) in the preconditioned heart (116 +/- 12 s, n = 6) compared to the control heart (86 +/- 10 s, n = 6, P<0.01), suggesting that decrease in contractile activity may be, in part but not completely, responsible for the reduction of MVO2. In contrast, the prolongation of T(1/2) was completely abolished by administration of a NO synthase inhibitor N omega-nitro-L-arginine in the high K+ arrested heart, demonstrating involvement of NO in the reduction of MVO2, presumably by suppression of mitochondrial respiratory chain. In conclusion, IPC reduces MVO2 during ischemia. The reduction of MVO2 in the preconditioned heart may be accounted for by decreased contractile activity and by depression of respiratory chain by NO.