The effect of steady-state moderate hypoglycaemia on human brain homeostasis has been studied with positron emission tomography using [U-11C]-D-glucose as tracer. To rule out any effects of insulin, the plasma insulin concentration was maintained at the same level under normo- and hypoglycaemic conditions. Reduction of blood glucose by 55% increased the glucose clearance through the blood-brain barrier by 50% and reduced brain glucose consumption by 40%. Blood flow was not affected. The results are consistent with facilitated transport of glucose from blood to brain in humans. The maximal transport rate of glucose from blood to brain was found to be 62 +/- 19 (mean +/- SEM) mumol hg-1 min-1, and the half-saturation constant was found to be 4.1 +/- 2.3 mM.