The ability of coronary endothelial cells in 14 day confluent cultures to metabolize glucose, palmitate, lactate and various amino acids was investigated. Under aerobic conditions, 99% of glucose, (5 mM) was degraded to lactate and only 0.04% was oxidized in the Krebs cycle. One percent of the glucose catabolized was directed into the hexose monophosphate pathway, but this fraction could be increased by 81% by 0.4 mM methylene blue. Glucose oxidation in the Krebs cycle was increased at glucose concentrations lower than 1 mM, or by the uncoupler 2,4-dinitrophenol. Oxidation to CO2 of palmitate (300 microM), lactate (1 mM), and glutamine (0.5 mM) was diminished in the presence of glucose (5 mM) by 80, 66, and 48%, respectively. These results demonstrate that coronary endothelial cells utilize exogenous glucose, at physiological concentration, predominantly for glycolytic energy production. The metabolic pattern is characteristic of the Crabtree effect. In these cells, glucose not only effectively suppresses the oxidation of the substrates lactate and palmitate, i.e. of substrates preferred by the whole heart, but also of glutamine, which is a major oxidative substrate for coronary endothelial cells. Absolute rates of substrate catabolism are low as compared to those of the beating heart indicating a low energy demand of coronary endothelial cells.