Patients with a colorectal malignancy were examined with a total body positron emission tomography, immediately following a brief infusion of 5-[18F]uracil. The radioactivity in normal liver tissue and in liver metastasis was monitored for two hours after infusion of the radiotracer. A kinetic model is described which permits calculation of the probable tissue concentration of 5-fluorouracil (FU) and its metabolites. Prerequisite for the model is that the time dependent plasma concentration of FU is known. It is hoped that such analysis will help predict therapeutic response, and permit evaluation of the kinetic consequences of different methods of drug application.