Clinical pharmacokinetic studies in our laboratory demonstrated that 2-fluoro-β-alanine (FBAL), the major catabolite of fluorouracil (FUra), has a prolonged elimination with an approximately 150-fold longer half-life than that of the unchanged drug in humans [Heggie et al.: Cancer Res. 47, 2203-2206 (1987)]. Recent studies have suggested that FUra catabolites, such as FBAL, may have a role in neurotoxicity and cardiotoxicity that may occur during FUra chemotherapy [Okada et al.: Acta Neuropathol. 81, 66-73 (1990)]. This study was undertaken to determine the kinetics and tissue distribution of FBAL in rats following iv bolus administration of radiolabeled FBAL. Plasma disappearance curves for FBAL could be described by the sum of three exponentials, with half-lives of 0.26, 12.1, and 8426 min. Radioactivity, consisting mainly of FBAL-bile acid conjugates, was excreted in bile within 30 sec of iv bolus administration of FBAL and continued throughout the experimental period at concentrations 10-100-fold higher than that of the corresponding plasma level. Urinary excretion, consisting mainly of free FBAL, represented the major pathway of elimination of FBAL, with 40% of the administered dose excreted within 24 hr and approximately 70% over 192 hr. Fecal excretion was a minor pathway of elimination of FBAL, with approximately 10% of the administered dose excreted over 192 hr. During the initial 30 min, the highest levels of tissue radioactivity were found in the kidneys, liver, spleen, lungs, and heart. Radioactivity was retained over longer time periods in the enterohepatic circulation, central nervous system, heart, and skeletal muscle. In summary, this study provides a comprehensive analysis of distribution and excretion of FBAL in plasma, bile, urine, and tissues in rats. The distribution and accumulation of FBAL and its metabolites may be important in understanding the sites and pattern of FUra toxicities.