The effects of hyperglycemia on myocardial glucose metabolism were investigated in seven healthy male subjects (age 24±4 yr). I6-C4qGlucose and IU-'3Cilactate were infused as tracers. Circulating glucose was elevated to two hyperglycemic levels using a clamp technique for 1 h at each level. The mean arterial glucose concentration was 4.95±0.29 (control), 8.33±0.31 and 10.84±0.60 jumol/ml, respectively. Glucose extraction increased significantly from control (0.15±0.13 gsmol/ ml) during each level of the glucose clamp (0.28±0.12, P < 0.02, and 0.54±0.14 ;tmol/ml, P < 0.005, respectively). Myocardial production of '4CO2 showed that during control 9±10% of exogenous glucose was oxidized immediately upon extraction. Despite a significant increase in the amount of exogenous glucose oxidized with level II hyperglycemia, it represented only 32±10% of the glucose extracted.[I3C]Lactate analysis showed that the myocardium was releasing lactate; during control 40±30% of this lactate was derived from exogenous glucose and during hyperglycemia this value increased to 97±37% (P < 0.005). Thus, these data show that during short-term hyperglycemia, myocardial glucose extraction is enhanced. However, despite increases in exogenous glucose oxidation and the contribution of exogenous glucose to lactate release, the majority of the extracted glucose (i.e., 57%) is probably stored as glycogen. (J. Clin. Invest. 1990Invest. . 85:1648Invest. -1656.) myocardial metabolism * glucose oxidation * lactate release * glycogen * free fatty acids