The tissue autoradiographic method for the measurement of regional cerebral blood flow (rCBF) in animals was adapted for use with positron emission tomography (PET). Because of the limited spatial resolution of PET, a region of interest will contain a mix of gray and white matter, inhomogeneous in flow and in tracer partition coefficient (lambda). The resultant error in rCBF, however, is less than 4%. Although the tissue autoradiographic method requires a monotonically increasing input function to ensure a unique solution for flow, the PET adaptation does not, because of an additional integration in the operational equation. Simulation showed that the model is accurate in the presence of ischemia or hyperemia of the gray matter. Inaccuracy in timing of the arterial input function will result in large errors in rCBF measurement. Propagation of errors in measurement of tissue activity is largely independent of flow, reflecting the nearly linear flow compared with activity relationship.