A system for measuring the rate of transport of dehydroascorbate into human red blood cells shows Michaelis-Menten type kinetics with substrate inhibition at levels above 150 muM DHA. The addition of sugars impairs this transport in the diminishing hierarchy D-glucose, D-mannose, D-xylose, D-galactose, L-lyxose, D-araboascorbate, L-sorbose and 2-deoxy-D-ribose. The effect of glucose on transport of ascorbate is marked at physiological levels. Transport of DHA is accelerated by copper ion and allows dehydroascorbate to move against a concentration gradient. The evidence supports the hypotheses proposing that hyperglycemia will impair the intracellular availability of vitamin C.