A validated, reversed-phase, isocratic high-performance liquid chromatographic method for the simultaneous assay of diminazene aceturate, antipyrine (excipient) and diminazene impurities in pharmaceutical formulations is described. The chromatographic system consisted of a Lichrospher-60 RP-select B column with a mobile phase composition of acetonitrile-methanol-ammonium formate (pH 4.0, 20 mM) (10:10: 80 v/v/v) and UV detection at 254 nm. The method is specific, precise and accurate for the determination of diminazene in the presence of its manufacturing and degradation impurities with a limit of detection and quantification of 50 ng/ml and 10 microgram/ml (RSD<3.0%), respectively. The major manufacturing impurity [1-(4 amidino phenyl)3-(4 carbamoyl phenyl)-triazene] and a degradant (p-aminobenzamidine) of diminazene aceturate have been resolved and identified by liquid chromatography/electrospray ionization-mass spectrometry operated in a positive ion mode.