A new method for determining tiopronin (N-2-mercaptopropionyl-glycine) in pharmaceutical formulations was developed and evaluated using flow injection analysis with a spectrophotometric detector.. The method is based on a redox reaction in which tiopronin participates as a reductant, wherebis(bathocuproinedisulfonate)cuprate(II) is reduced to the yellow-orange compound bis(bathocuproinedisulfonate)cuprate(I). The absorbance of the resulting compound was measured at 483 nm. The univariate method was used to select optimal chemical parameters and parameters of the flow system, the linear dynamic range was determined in the concentration range from 3×10–7 mol L–1 to 3×10–5 mol L–1 . Using linear regression analysis, the equation y = 5016 x + 0.0019 was obtained with the correlation coefficient R 2 = 0.9998 and the limit of quantification (LOQ) of 3×10–7 mol L −1 and the limit of detection (LOD) of 9×10–8 mol L –1 . Performing the evaluation of the method its accuracy was established within a range of recovery from 91.07 to 103.55 %. Aside from lactose, sucrose, citric acid, tartaric acid and acetylsalicylic acid, other excipients of pharmaceutical preparations do not interfere when measuring the tiopronin signal. The method showed satisfactory reproducibility with a relative standard deviation of 1.42 % and allows for 60 analyses in triplicate, or a total of 180 analyses, during one hour. The application of the method did not result in the carryover of a signal, and the determined reach of the reaction is 92 %. The developed method was compared to the standard method according to the Pharmacopoeia and can be successfully applied for the determination of tiopronin in pharmaceutical formulations.