Adenine (ATP, ADP, AMP) and pyridine nucleotides (NADP+, NADPH, NAD+, NADH) concentrations have been determined by HPLC in the erythrocytes from five different mammalian species (pig, rat, mouse, rabbit and cow) and compared to those in human red blood cells. Two different extraction procedures have been used and the results obtained are compared and discussed. A good correlation between the different abilities of the erythrocytes of the six species to utilize glucose and the NAD+/NADH ratio was found, with high NAD+/NADH ratio in the red blood cell of the species with high glucose utilization rates. The levels of all the glycolytic enzymes and some of the pentose phosphate shunt enzymes were also determined.