The present invention provide a method for measuring and assessing the capacity in an individual subject to metabolize a variety of fluorouracil drugs, such as 5-fluorouracil, which are degraded in the pyrimidine metabolic pathway, and a preparation useful for the measurement and assessment. The present invention can be carried out by: administering a preparation for determining pyrimidine metabolic capacity comprising as an active ingredient a pyrimidine compound or pyrimidine. metabolic compound that acts as a substrate for a pyrimidine metabolizing enzyme, in which at least one of the carbon, oxygen, and nitrogen atoms is labeled with an isotope and assessing in vivo pyrimidine metabolic capacity based on the behavior of an excreted metabolite.
