1. The metabolic fate of a new antitumour agent, 1-hexylcarbamoyl-5-fluoro [6-14C]uracil (14C-HCFU) in rats after oral administration was compared with that of 5-fluoro[6-14C]uracil (14C-FU). 2. Tissue radioactivity reached a max. 1 to 3 h after administration of 14C-HCFU and 0.5 h after 14C-FU. 3. Both drugs were excreted rapidly, mostly in urine. Expired 14CO2 from 14C-HCFU was significantly less than that from 14C-FU. 4. Unchanged FU was not detected in plasma 3 h after administration of 14C-FU, whereas FU was detected in plasma 5 h after 14C-HCFU. The pyrimidine ring of 14C-HCFU might be degradated more slowly than that of 14C-FU. 5. 1-(5-Carboxypentylcarbamoyl)-5-fluorouracil and 1-(3-carboxypropylcarbamoyl)-5-fluorouracil were identified as the major urinary metabolites of 14C-HCFU.