A simple isocratic stability-indicating high-performance liquid chromatographic method with UV detection using thymine as an internal standard is developed. The method is validated and the degradation products are determined. The method is applied for the assessment of the stability of 5-fluorouracil in rat caecal content as a simulated colon medium under anaerobic conditions. The drug decomposes under acidic, alkaline, thermal, and oxidative stress. The drug is highly susceptible to acidic, alkaline, and oxidative hydrolysis as compared to alkaline conditions. Separation of the drug from major and minor degradation products is successfully achieved on a C(18) analytical, micro-bondapak column. The detection wavelength is 260 nm. The method is validated, and the response is found to be linear in the drug concentration range of 0.1-2.0 microg/mL. The high linearity of the standard calibration curve of 5-fluorouracil in the rat content is found to be R(2) = 0.998 in the concentration range from 0.5 to 5 microg/mL. No degradation occurred after incubation of 5-fluorouracil in the rat caecal contents. The standard deviation and coefficient of variation values for intra- and inter-day precision study exhibit acceptable accuracy and precision data throughout the concentration range investigated.