An investigation has been carried out into the anodic differential-pulse polarographic and cathodic stripping voltammetric behaviour of the anticancer drug 5-fluorouracil at mercury indicator electrodes. The anodic peak, corresponding to the formation of a mercury salt, has been used for the determination of the drug in formulations with a relative standard deviation of 0.5%. The cathodic stripping voltammetric behaviour of this mercury salt can be used to monitor 10^–7–10^–8M concentrations of 5-fluorouracil in a borax-HClO₄ buffer, pH 7.8. The interference of Cl^– with the cathodic stripping signals is only evident at concentrations of Cl^– four orders of magnitude higher than the concentration of the drug and uric acid was found not to interfere at up to equal concentrations of 5-fluorouracil. The cathodic stripping method has been applied to the determination of 5-fluorouracil in human serum.