The problem of any organism surviving under anaerobic conditions is to convert the potential energy of its substrate molecules into biologically useful energy without having to use molecular oxygen. The resistance of the fetal and newborn mammal to hypoxia might depend upon some unique chemical reaction, upon quantitative differences in the rates of certain enzymatic reactions common to fetal and adult tissues, or upon a lower rate of cellular metabolism in fetal tissues. These experiments with human fetal tissues favor the second of these three possibilities. Tissue slices of human liver, cerebral cortex, brain stem, heart, lung, kidney and skeletal muscle were incubated in a medium containing C14 labeled glucose, fructose, pyruvic acid or acetic acid. These experiments showed that there is a fourfold greater rate of glycolysis in fetal liver midway through gestation under anaerobic conditions than under aerobic conditions. In contrast, the rate of lipogenesis under anaerobic conditions was only one-tenth as great as the rate under aerobic conditions. Experiments revealed that after a slice of liver or cerebral cortex had spent a full hour in complete anoxia, its ability to synthesize fatty acids and to oxidize substances by way of the Krebs cycle was essentially unimpaired. All of the experiments were designed to test the maximal activity of the enzyme systems involved. An excess of substrate was provided so that the activity of the cellular enzymes, and not the concentration of the substrate, was the ratelimiting factor. These experiments provide an estimate of the upper limit of the metabolic activity of the tissue in oxygen and in nitrogen. They do not provide an estimate of metabolic rates in vivo. Observations on human tissues near term showed that the rate of aerobic lipogenesis was much less than that observed in tissues of younger fetuses. This was equally true with labeled glucose, pyruvate and acetate as substrates. The pattern of lipogenesis is comparable to that in the liver of a rat 24 hours after parturition and not to the pattern which obtains in the rat just before birth. The rate of anaerobic lipogenesis is substantially less than that under aerobic conditions. These observations may be correlated with the idea that the newborn rat is more immature than the newborn human. These experiments indicate that lipogenesis is of no significance in the ability of the human fetus one half way through gestation to withstand hypoxia, for under anaerobic conditions lipogenesis is decreased whereas glycolysis is increased.