Tumor cells are highly dependent on glucose metabolism. The energetic homeostasis of these cells is critically controlled by the high energy phosphate buffer of creatine/phosphocreatine (Cr/PCr). High glycolytic flux induced by hyperglycaemic conditions provokes in many normal cells high rates of ROS generation. Recently, it was demonstrated that the creatine kinase activity controls the rate of ROS production by mitochondria in hyperglycaemia. The role of the Cr/PCr buffer under high and normal glucose supply has been poorly investigated in tumor cells. In this study we investigate the role of creatine on the proliferation and/or bioenergetic mitochondrial parameters of the human hepatoma HepG2 cells. In hyperglycaemia (40 mM glucose) creatine caused an almost twice increase in HepG2 MTT absorbance. Creatine had no effect in MTT absorbance of HepG2 cells under normoglycaemia (5 mM glucose). In digitonin permeabilized HepG2 cells the rate of mitochondrial oxygen consumption induced by Cr/ATP was four fold stimulated. The respiratory control ratio of this preparation was about 6. These results suggest that HepG2 mitochondria have accessory creatine kinase activity that may be compromised to control the rate of oxygen consumption and possibly ROS production. Further studies are in course to establish the involvement of creatine kinase activity on ROS generation and under limiting glucose conditions.