In contrast to thymine and 5-fluorouracil (FUra) which were cleared from the bloodstream within 2-4 h after their i.p. administration (200 mumol/kg) to rat, (E)-5-(2-bromovinyl)uracil (BVUra) maintained a concentration of 50-70 microM for at least 6 h and was still present in the plasma 24 h after its administration. In vitro experiments with rat liver extracts indicated that BVUra was not a substrate but an inhibitor for the reductive step in pyrimidine degradation catalyzed by dihydrothymine dehydrogenase. Kinetic and dialysis experiments suggested that BVUra was an irreversible inhibitor of this enzyme. The binding of BVUra to the enzyme depended on the presence of reduced nicotinamide adenine dinucleotide phosphate in the reaction mixture. Dihydrothymine dehydrogenase activity was also inhibited in the dialysed 105,000 X g supernatant fraction of livers from rats that had previously been treated with BVUra. Such inhibitory effects also occurred in vivo; previous administration of BVUra increased the plasma half-lives of thymine and FUra by 10- and 5-fold and their area under the curve by 9- and 8-fold, respectively. The effect of BVUra on the antitumor activity of FUra was evaluated in DBA/2 mice inoculated with 10(6) P388 leukemia cells. The mean survival times for the control and FUra-treated mice (5 mg/kg at 1, 3, 5, and 7 days after tumor cell inoculation) were 9.7 and 12.4 days, respectively. When BVUra (200 mumol/kg) was administered 1 h before each injection of FUra, the mean survival time was extended to 17.1 days. BVUra alone did not affect the mean survival time. When the dose of FUra was increased to 20 mg/kg, the mean survival time was 15.3 days; upon a preceding injection of BVUra the mean survival time decreased to 9.2 days. The latter effect probably resulted from an increased toxicity of FUra. Similar results were obtained if FUra was replaced by 5-fluoro-2'-deoxyuridine and BVUra by (E)-5-(2-bromovinyl)-2'-deoxyuridine. The enhancement of both the antitumor and toxic effects of FUra by BVUra were most probably due to an inhibition of FUra degradation, since, like in rats, BVUra increased the plasma half-life of FUra in DBA/2 mice. Hence BVUra appears to be an interesting compound, increasing the potency of FUra by decreasing its degradation.