Two metabolites of 5-fluorouracil (FU), monofluoroacetic acid (FA) and alpha-fluoro-beta-alanine (FBAL), were continuously administered into the left ventricle of the brain in cats for up to 1 month to investigate the mechanism of neurotoxicity of FU and its derivatives. The cumulative doses of FU and FBAL over a 1-month period were 1.5-45 mg (20 cats) and 0.2-4.8 mg (21 cats), respectively. As controls for each experimental group, acetic acid (AA) and beta-alanine (BAL) were administered. In terms of survival time in relation to the cumulative dose and molecular weight, FBAL was more toxic than FA. Neuropathologically, two types of change, vacuoles and necrosis/softening-like change, were found. The vacuoles were 20-50 microns in diameter, and distributed mainly in the cerebellar nuclei, white matter and the tectum and tegmentum of the brain stem in both experimental groups. Electron microscopically, these vacuoles were due to splitting of the myelin intraperiod line or separation between the axon and the innermost layer of myelin. Necrosis/softening-like change occurred preferentially in the FBAL group and was located symmetrically in the superior and inferior colliculi, oculomotor nuclei and thalamus. Both types of neuropathological change, especially those in the FBAL group, were similar to those found in cats orally administered with FU and its derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)