Department ofNuclear Medicine, Heinrich-Heine-University, Diisseldorf, Forschungszentrum Juelich; and Diabetes Research Institute, Dusseldo, j Germany three-compartment model and previously reported values for K1, k2 and k3 determined in skeletal muscle tissue by FDG PET (7— 9)(Table 1). The rate constant k.@, indicating the dephosphorylation of FDG-6-P, was set to zero because it is negligible for times up to 45 mm postinjection (8, 10). The fractional volume (FV), which reflects the volume within the muscle tissue that contains extracel lular but not intracellular tracer activity, was fixed to 3.5% as measured by PET with †50-CO (11). The time t@ describes the time difference between the appearance of the injection bolus in the sampled plasma input and the muscle tissue that is measured in the PET scanner. The time t@ was set to 0.0 s in the simulations of the influences of input and tissue sampling and of Pt.