Validation is a key step in the creation of an analytical method after its development, and it is defined as the process of proving whether the method is suitable for the intended purpose. Analytical method validation is conducted in the laboratory by testing performance characteristics of the method, such as selectivity, specificity, linearity, working range, precision, trueness, limit of detection and quantification, and robustness. Depending on the purpose of the analytical method, it is not necessary to examine all performance features, but only selected ones. The main goal of this work is to validate the LC-MS/MS method for the determination of N-nitroso-hydrochlorothiazide in hydrochlorothiazide active substance. The experimental part of the work covered determination of all performance characteristics and was conducted in the Quality Control Department, Pliva Croatia d.o.o. The method achieved good linearity (R^2=1,000) and met the criteria for precision (RSD ≤ 20%) and accuracy (RSD ≤ 25%). The limit of quantification (LOQ = 0,60 ng/ml) as well as the working range of the method (0,60 ng/ml – 50 ng/ml) are defined. In addition, the stability of the solutions of N-nitroso-hydrochlorothiazide standard and hydrochlorothiazide active substance and the robustness of the method were evaluated. The validation results confirmed the acceptability of the method for the determination of N-nitroso-hydrochlorothiazide in hydrochlorothiazide active substance was confirmed, given that the obtained values of performance characteristics are within the given limits of acceptability. This enables the application of the method in the daily routine work of the laboratory.