Glycolysis and oxidative metabolism were assessed in washed human platelets incubated in a Ca++ and Mg++ free Krebs-Ringer bicarbonate buffer, pH 7.4 at 37° C for 1 hour. Glycolytic rate was 45-65 per cent lower under aerobic than anaerobic conditions. Glycolysis was decreased further when albumin was present in the incubation medium and under these conditions glucose uptake, glycogen utilization and lactate production were 21.4, 15.8 and 78 µmoles/hr. per 1011 platelets, respectively. The oxidation of 6-14C glucose was 0.39 µmoles/hr. per 1011 platelets and of U-14C palmitate 0.19 µmoles/hr. per 1011 platelets, and the rate of oxidation of either substrate was increased in the absence of the other. The uncoupling agent, dinitrophenol, stimulated oxidation of glucose to a much larger extent than fatty acid. It is concluded that both glycolysis and oxidative phosphorylation are important to platelet energy metabolism, that either pathway may compensate for decreased activity of the other and that, under conditions of metabolic stress, glucose is preferred to fatty acids as a substrate for oxidative metabolism.