The unidirectioal transfer of D-glucose from blood to parietal cortex tissue of the brain of awake rats was measured by single intravenous injection of tracer glucose, as well as by single intracarotid injection according to the method of Oldendorf. The maximal unidirectional blood-brain glucose transfer rate (Tmax) was 407 mumol (100 g)-1 min-1 when measured by intravenous injection, and 352 mumol (100 g)-1 min-1 when measured by intracarotid injection. The half-saturation constants (Km) were 7.8 mM and 16.8 mM, respectively. The comparison shows that the two methods give similar results when cerebral perfusion is assessed accurately.