The operational equation for the double-label deoxyglucose method described in the following paper requires the knowledge of the rate constants for transfer of fluorodeoxyglucose across the blood-brain barrier (K1* and K2*), and those for phosphorylation of fluorodeoxyglucose (K3*) and dephosphorylation of fluorodeoxyglucose-6-phosphate (k4*). These rate constants were determined in anesthetized rats by external coincidence counting. Radioactivity in parietal brain was measured for a 110 min experimental period after a bolus injection of 18F-labeled fluorodeoxyglucose. Apparent rate constants were obtained by fitting the resulting tissue radioactivity curves to the tissue radioactivity function of the deoxyglucose model modified to take into account the dephosphorylation of fluorodeoxyglucose-6-phosphate. The apparent fluorodeoxyglucose rate constants in rat brain are K1* = 0.195 ml g-1 min-1, k2* = 0.379 min-1, k3* = 0.088 min-1, and k4* = 0.009 min-1.