The pharmacokinetics of phthorafur-2-14C (Ph) was investigated after its intravenous injection to rats with Walker carcinosarcoma. The blood plasma level of Ph-2-14C and its metabolites proved to decrease in to a three-phase process. The content of the agent in the tissues decreased in the following sequence: the kidney, small intestine, tumour, stomach, muscle, heart, liver, lungs, spleen, brain and fat. The tumour was observed to contain Ph-2-14C and endogenous metabolite 5-phthoruracil-2-14C. Excretion of the agent continued for 48 hrs, 52.2% of the administered dose being eliminated via the urinary tract, 30% as 14CO2, and 0.8% in feces.