Using sequential injection analysis with a spectrophotometric detector, a new kinetic method for the determination of tiopronin (N-2-mercaptopropionyl-glycine, MPG) in pharmaceutical preparations was developed and validated. The method is based on a redox reaction in which tiopronin reduces the yellow bis(bathocuproinedisulfonate)cuprate(II) to a stable bis(bathocuproinedisulfonate)cuprate(I), characterized by a yellow-orange color, with an absorption maximum at a wavelength of 483 nm. The absorbance of the formed complex was continuously measured using a UV-Vis spectrophotometer, with measurements performed in triplicate. The chemical and flow parameters of the system were optimized using the univariate method, based on which a calibration curve was constructed to determine the linear dynamic range of the method. Regression analysis yielded the equation y = 5897x + 0.0176, with a linear range of analyte concentrations from 8.0 × 10 -7 to 6.0 × 10 -5 mol L -1 . The limits of detection and quantification were determined as 2.54 × 10 -7 mol L -1 and 8.48 × 10 -7 mol L -1 , respectively. The method was evaluated in terms of accuracy, efficiency, and precision, including reproducibility, analysis frequency and reaction yield. The accuracy of the method was confirmed with efficiency ranging from 100.70% to 110.81%. The reproducibility was satisfactory, with a relative standard deviation of 4.66%. In one hour, 12 triplicate analyses can be performed, resulting in a total of 36 analyses. The reaction yield was 90.50%. Apart from acetylsalicylic acid and high concentrations of boric acid, none of the commonly present excipients in pharmaceutical products interfered with the method. The developed method was successfully applied for the determination of tiopronin in the pharmaceutical preparation Captimer®, and the obtained results showed a high correlation with the standard Pharmacopoeial method.