Antitumor activity of metabolites of 1-hexylcarbamoyl-5-fluorouracil (HCFU) and related compounds was examined in vivo and in vitro. Carboxypentyl and carboxypropyl carbamoyl derivatives of 5-fluorouracil (FU) were moderately active against L1210 by oral administration but less active by intraperitoneal administration. However, 5-hydroxy-and 5-oxo-hexyl carbamoyl derivatives of FU were markedly or moderately active against the leukemia by both oral and intraperitoneal administrations. Therapeutic ratios for the metabolites were less than that for HCFU by both oral and intraperitoneal administrations. Metabolites of HCFU had growth inhibitory activity against L5178Y in vitro similar to alkylcarbamoyl derivatives of FU. Activity of metabolites was lower than that of FU and higher than that of HCFU in vitro. It means that HCFU is converted into FU through active intermediate metabolites in vivo.