A kinetic spectrophotometric method for the determination of ranitidine hydrochloride, based on its inhibitory effect on Hg(II) catalyzed substitution of cyanide ion, by 4-cyanopyridine in hexacyanoferrate(II) is described. Ranitidine hydrochloride ions form strong complexes with Hg(II) catalyst which is used as the basis for its determination at trace level. The progress of reaction was monitored, spectrophotometrically, at 477nm (λmax of [Fe(CN)5CNpy]3−, complex) under the optimum reaction conditions at: [Fe(CN)64-] = 5 × 10-3 M, [4-CNpy] = 2.5 × 10-4 M, [Hg2+] = 2 × 10-5 M, pH = 2.8 ± 0.02, I = 0.02 M (KNO3) and temperature = 25±0.1°C. A linear relationship obtained between absorbance (measured at 477nm at different times) and inhibitor concentration, under specified conditions, has been used for the determination of [ranitidine hydrochloride] in the range of 0.2 – 2.0 × 10−5M with a detection limit of 5.2 × 10−7 M. The standard deviation and percentage relative standard deviation have been calculated and reported with each datum. A most plausible mechanistic scheme has been proposed for the reaction. The values of equilibrium constants for complex formation between catalyst–inhibitor (KCI), catalyst–substrate (Ks) and Michaelis–Menten constant (Km) have been computed from the kinetic data. The influence of possible interference by major cations and anions on the determination of ranitidine hydrochloride and their limits has been investigated