The use of a combination of different drugs in postoperative analgesia extends the time of analgesia, makes it more efficient and allows the use of lower drug doses, which leads to less risk of side effects and drug dependence. The aim of this study was to develop and validate an HPLC method to determine the stability of fentanyl citrate and bupivacaine hydrochloride mixtures in standard infusion solutions of 0.9% sodium chloride and 5% glucose. AFTER OPTIMISATION, THE HPLC METHOD PARAMETERS WERE AS FOLLOWS: LiChrospher 100 CN, 250×4 mm (10 µm) column; mobile phase: mixture of acetonitrile and phosphate buffer at pH 2.8 (3:7, V/V) with addition of 0.08 g/l potassium chloride; flow rate: 1.5 ml/min; column temperature: 30°C; spectrophotometric detection at 210 nm. Development of the method involved checking the impact of acetonitrile and KCl concentrations in the mobile phase and choosing the internal standard. Method validation included determining the specificity of the method, its accuracy, linearity, precision, repeatability, limits of detection and quantification. The retention times of bupivacaine hydrochloride, fentanyl citrate and procaine hydrochloride, used as an internal standard, were approximately 10 min, 15 min and 5 min, respectively. Method validation confirmed its selectivity, accuracy and precision. The average values of the variation and accuracy coefficients were 0.70% and 99.02% for bupivacaine hydrochloride, and 1.76% and 104.53% for fentanyl citrate. The intermediate precision values were 1.25% for bupivacaine hydrochloride and 1.52% for fentanyl citrate.