We have determined the rate constants (ki*) of 18F-fluorodeoxyglucose (FDG) in the unlesioned baboon brain, for use in positron emission tomography (PET) measurements of glucose utilization. In contrast to earlier reports, we used a radiosynthesis which guarantees production of FDG essentially uncontaminated by fluorodeoxymannose, and an improved determination of ki* by (1) direct measurement of the time-shift between bolus arrival in femoral arterial plasma and brain, (2) rapid initial PET frames, and (3) extended data acquisition (up to 180 min). Young adult baboons were studied under anesthesia with either phencyclidine or etomidate. The FDG time-activity curves obtained from temporal grey matter showed a consistent decline after about 80 min, indicating true product loss. Three-compartment modelling was performed for increasing fitting intervals (20-120 min) with both a 5-parameter (K1*-k4*, and vascular volume (Vo)) and a 4-parameter (K1*-k3*,Vo) model. With the latter, both the calculated FDG net clearance ((K* = K1*.k3*/(k2* + k3*)) and the fitted kinetic constants were dependent on fitting interval, i.e., they showed sustained unstability. With the former, the constant k4*, which presumably represents dephosphorylation, was overestimated and unstable for short fitting times (presumably due to heterogeneous brain compartments in the sample tissue), but stabilized at approximately 0.01 min-1 for fitting times > or = 80 min; K1*-k3* and K* were also stable after this time. These findings were identical for both anesthetic regimen. Thus, in the anesthetized baboon, the FDG ki* values can be reliably determined based on an adequate PET acquisition paradigm and with a model that incorporates k4* and > or = 80 min time-activity data.