71 malignant and 20 benign human tumors were tested as to their capability of oxygen utilization using endogenous substrates or either glucose or glutamine or a combination of these two energy substrates. It was found that tumor oxygen uptake did not differ significantly from that of peritumoral tissues excised in the same chirurgical procedure. No difference was found in glucose and glutamine uptake between tumoral and peritumoral tissues, but it was shown that glutamine decreases glucose uptake while glucose increases glutamine uptake in tumors. It was demonstrated that the radioactive label from glucose could be found in CO2 showing that there is not a complete uncoupling of glycolysis and tricarboxylic acids cycle in human tumors. It is proposed that the typical increased lactate production in tumors might be due to inhibition of reduced equivalent shuttles between cytoplasm and mitochondria concomittant with a non-controlled phosphofructokinase.