Rate constants and fractional metabolic rates were estimated for the stereoisomers [18F]2-fluoro-2-deoxyglucose (FDG) and [18F]2-fluoro-2-deoxymannose (FDM) in 5 male baboons using positron emission tomography. Each animal, serving as its own control, was injected with both compounds under controlled physiological conditions. Results indicate that there is a 20% reduction in apparent cerebral metabolic rates for glucose when FDM is used as the analogue. This suggests that as supplies of 18O become depleted, the presence of FDM as an impurity should warrant consideration in the choice of alternatives to no-carrier-added FDG synthesis.