Quantification of myocardial glucose uptake by positron emission tomography with [¹⁸F]fluorodeoxyglucose (FDG) requires the "lumped constant" (LC), which corrects the difference of affinity between glucose and FDG to glucose transporters and phosphorylating system. Since LC was introduced, it has been considered to be constant. However, this has recently been questioned. To elucidate the constancy of LC by other than radioisotope techniques, the accumulation rate of sugar phosphates (d[SP]/dt) was measured in isolated, perfused rat hearts by ³¹P NMR spectroscopy with 2-deoxyglucose (DG). We postulate  as the affinity of DG to transporters and the phosphorylating system relative to that of glucose. Theoretically,  is equivalent to LC. We determined  by measuring d[SP]/dt at DG concentration ([DG]) = 10, 7, 5, and 3 mmol/l, keeping the total of glucose concentration ([glucose]) and [DG] to 10 mmol/l. When the glucose uptake was enhanced by insulin (10 mU/ml) or stunning, calculated  was reduced (insulin stimulated, 0.15; stunning, 0.19) compared with the control (0.59). These results indicate that LC can be evaluated by methods without radiolabeled tracers and is smaller when glucose uptake is augmented.