NMR-spectroscopy (31P-NMR, 23Na-NMR) has been used to characterize the energy metabolism of cultured endothelial cells of the pig aorta and to determine the energy requirements of the Na-K-ATPase relative to total cellular energy consumption. Endothelial cells exhibited high concentrations of creatine phosphate and cardiac microvascular creatine kinase (CK) was of the BB-CK subtype. BB-CK can therefore be used as a marker-enzyme of endothelial cells in muscular organs. The transmembrane flux-rate of sodium was 117 mumol Nalmin/mg prot in the steady state at an intracellular sodium concentration of 25 mmol/l. Combining microcalorimetric measurements with 23Na-NMR data revealed that the energy requirement of endothelial Na-K-ATPase despite the high surface to volume ratio comprises only 3-5% of total cellular energy consumption.