In humans, 80-90% of the administered dose of 5-fluorouracil (5-FU) is degraded by dihydropyrimidine dehydrogenase (DPD), the initial rate-limiting enzyme in pyrimidine catabolism. In a previous report, the concentration ratio of uracil (U) and its dihydrogenated metabolite dihydrouracil (DHU) in human plasma before 5-FU treatment was found to be useful as an index of optimal 5-FU dosage, and was reported to be helpful in identifying patients with metabolic deficiency and anticipated risk of toxicity. We established an assay method for U, DHU, 5-FU and 5-fluoro-5,6-dihydrouracil (5-FDHU) in human serum and rat plasma using reverse-phase HPLC. After deproteinization of serum or plasma with perchloric acid, the supernatant was injected into HPLC with a column switching device and quantitated by UV detection. Separation was achieved by 3 columns whose characters were different and a mobile phase of 5 mmol/l sulfuric acid. In normal human serum, concentrations of U and DHU were 7.67-12.1 ng/ml and 70.4-103 ng/ml respectively, giving a DHU/U concentration ratio of 7.59-11.8. We determined U and DHU concentrations in rat plasma administered with 5-FU, and examined the correlation between DHU/U concentration ratio and DPD activity in the liver. Consequently, it was confirmed that U and DHU concentrations in rat plasma and DHU/U ratio were affected by 5-FU treatment. Also, there was a weakly significant, positive correlation between plasma DHU/U ratio and liver DPD activity (r = 0.44, p = 0.035). We expect that the serum DHU/U ratio obtained easily with this method can be utilized as a new index for safety with 5-FU treatment.