Pentobarbital anesthesia (40 mg kg-1) was accompanied by a 50% decrease of blood flow and a 40% decrease of unidirectional blood-brain glucose transfer in the parietal cortex of the rat brain. The correlation was explained by a decrease of the number of perfused capillaries. The maximal transport capacity, Tmax, decreased from 409 to 235 mumol 100 g-1 min-1 and the half-saturation constant, Km, from 8.8 to 4.9 mM. At 8.3-8.7 mM-glucose in arterial plasma, the transfer constant (clearance) for unidirectional blood-brain transfer decreased from 0.195 +/- 0.011 in awake rats to 0.132 +/- 0.005 ml g-1 min-1 in anesthetized rats. Half of the decrease was due to less complete diffusion-limitation of glucose uptake at the low plasma flow rate in brain, the other half to the decreased Tmax.