Novel, sensitive and rapid electrochemical methods for the determination of phenothiazine and azaphenothiazine derivatives were developed. A boron-doped diamond (BDD) electrode was used for electrochemical oxidation of levomepromazine, promazine and prothipendyl. The electrooxidation of these substances demonstrated reversible peaks of oxidation at the potential range 0.55 - 0.75 V vs. SCE. Examining the influence of scan rate allowed is to demonstrate that the currents registered typical of the diffusion-controlled process. Determinations of the studied analytes were carried out by means of a square wave voltammetry (SWV) method and a differential pulse voltammetry (DPV) method. Linear ranges of determination with the use of the BDD electrode and the SWV method were obtained in the ranges: from 4 × 10(-7) to 1.38 × 10(-4) mol L(-1) for levomepromazine, from 4 × 10(-7) to 1.17 × 10(-5) mol L(-1) for promazine and from 4.95 × 10(-7) to 4.54 × 10(-5) mol L(-1) for prothipendyl. The influence of interferences on the voltammetric signal of the studied analytes was also checked. The proposed procedures were used for quantitative determination of the studied compounds in pharmaceutical preparations. The measurements showed high accuracy. The recovery values obtained ranged from 98.52 to 99.57%. The developed procedures were compared with pharmacopoeial reference methods.