19F NMR spectroscopy has been used to study further the metabolism of 5-trifluoromethyl-2'-deoxyuridine (trifluridine; F3TdR). The synthesis and characterization of alpha-trifluoromethyl-beta-alanyl glycine (F3MBAG), a putative new metabolite of F3TdR, are now reported. This study describes ex vivon and in vivo detection of F3MBAG and other previously reported metabolites of trifluridine, using 19F NMR spectroscopy, in male BALB/C mice bearing EMT-6 tumors. A parallel 19F NMR spectroscopic study was also performed on rats dosed with F3TdR, to observe the qualitative pattern of F3TdR metabolism in another species. Unexpectedly, 5-trifluoromethyl-5,6-dihydroxyuracil (DOHF3T), alpha-trifluoromethyl-beta-ureidopropionic acid (F3MUPA) and fluoride, which result from the metabolic degradation of F3TdR and which were detected in various biological samples from mice dosed with F3TdR, could not be identified in rat urine or in homogenized tissue extracts. The presence of these metabolites in intact tissues is uncertain since in this study 19F NMR spectroscopy of these samples always displayed a broad resonance "hump" across the range of chemical shifts that would encompass these metabolites. No clear explanation for the loss of spectroscopic resolution in this region has been rationalized. N-Carboxy-alpha-trifluoromethyl-beta-alanine (F3MBA-CO2), alpha-trifluoromethyl-beta-alanyl alanine (F3MBAA) and N-acetyl-alpha-trifluoromethyl-beta-alanine (Ac-F3MBA) were synthesized and characterized, but were not detected as metabolites in any of the biological specimens examined.