The antineoplastic agent 5-azacytidine, one of the antimetabolites, has been studied systematically using polarography, cyclic voltammetry and coulometry in the buffered aqueous medium pH 6.4 at 5 degrees C, conditions for maximum stability of the 1,3,5-triazine ring. The reduction mechanism proposed is an overall two-electron process, where the reducible group is the N(5)=C(6) double bond. Differential pulse polarography has been employed for its quantitative analysis at the optimum found conditions, and a detection limit of 0.4 mu M was estimated. As both compounds appear together in biochemical reactions when 5-azacytidine is used in cancer chemotherapy, different mixtures of 5-azacytidine + cytidine have also been investigated at micromolar level and a mean error of about 2% was obtained.