We estimate the influence of five 5-hydroxytryptamine receptor (5-HT 3 ) antagonists on the activity of dihydropyrimidine dehydrogenase (DPDase), the rate-limiting enzyme in 5-fluorouracil (5FU) metabolism. The activity of DPDase from the rat liver was compared in the cytosol mixture of 5FU incubated with or without each of five 5-HT 3 antagonists. DPDase activity was not altered in the presence of any 5-HT 3 antagonist studied here. It may be inferred from these results that the any 5-HT 3 receptor antagonist examined in this study has little or no effect on fluorouracil catabolism.