A sensitive voltammetric method has been developed for the determination of several phenothiazine drugs including chlorpromazine, promazine, perphenazine and fluphenazine. The method is based on catalytic effect of methylene blue as a homogeneous mediator on the oxidation of the drugs at the surface of a glassy carbon electrode. Cyclic voltammetric measurements showed that, the catalytic current of the system depends on the concentration of the drugs. Optimum pH values for the voltammetric determination of the drugs was 8.6. Under the optimized conditions, the calibration curves were linear in the concentration range of 5.0-70.0 and 70.0-210.0 for chlorpromazine, and 5.0-70.0 and 70.0-120.0 µM for promazine, perphenazine and fluphenazine. The detection limit (signal to noise ratio of 3) were 2.25, 2.25, 2.08 and 2.09 µM for chlorpromazine, promazine, perphenazine and fluphenazine, respectively. The chronoamperometry was used to determine the catalytic rate constant for the catalytic reaction of methylene blue with chlorpromazine. The proposed method was used for the selective determination of phenothiazines including chlorpromazine, perphenazine, promazine, and fluphenazine in human urine and serum and some pharmaceutical products including tablets and ampoul using the standard addition method.