Although numerous positron emission tomography (PET) studies with (18) F-fluoro-deoxyglucose (FDG) have reported quantitative results on cerebral glucose kinetics and consumption, there is a large variation between the absolute values found in the literature. One of the underlying causes is the inconsistent use of the lumped constants (LCs), the derivation of which is often based on multiple assumptions that render absolute numbers imprecise and errors hard to quantify. We combined a kinetic FDG-PET study with magnetic resonance spectroscopic imaging (MRSI) of glucose dynamics in Sprague-Dawley rats to obtain a more comprehensive view of brain glucose kinetics and determine a reliable value for the LC under isoflurane anaesthesia. Maps of Tmax /CMRglc derived from MRSI data and Tmax determined from PET kinetic modelling allowed to obtain an LC-independent CMRglc . The LC was estimated to range from 0.33 +/- 0.07 in retrosplenial cortex to 0.44 +/- 0.05 in hippocampus, yielding CMRglc between 62 +/- 14 and 54 +/- 11 mumol/min/100 g, respectively. These newly determined LCs for four distinct areas in the rat brain under isoflurane anaesthesia provide means of comparing the growing amount of FDG-PET data available from translational studies. The lumped constant (LC) is a correction factor employed to estimate the glucose metabolic rate (CMRglc ) from the metabolic rate of a radioactive glucose analog such as (18) F-fluorodeoxyglucose (FDG). By combining FDG positron emission tomography (PET) with magnetic resonance spectroscopic imaging (MRSI) of glucose dynamics, we determined the FDG LC in the rat brain under isoflurane anaesthesia.