In the first part of this project a sensitive voltammetric method has been developed for the determination of captopril. The oxidation of captopril has been studied at a glassy carbon electrode by electrocatalytic effect of chlorpromazine as a homogenous mediator, using electrochemical impedance spectroscopy, cyclic and linear sweep voltammetry, chronoamperometry and as diagnostic and measurement techniques. Cyclic voltammetric investigation showed that, although captopril didn’t show any distinguishable anodic peak in the potential range 0.0 - 1.1 V vs SCE, but in the presence of captopril, proportionate concentration of it, the anodic peak current of chlorpromazine enhanced. Under the optimized conditions the catalytic oxidation peak current of captopril was linearly depend on the its concentration and allows the determination of captopril over the concentration range of 8.0×10 -6 – 1.0×10 -3 M captopril at pH 5.0, with a detection limit (LOD) of 4.9×10 -6 M. The relative standard deviation (RSD) of 0.66% for the determination of 4.0×10 -4 M captopril solution in 9 repeated measurements. The chronoamperometry was used to determine the catalytic rate constant and diffusion coefficient for the catalytic reaction of chlorpromazine with captopril. The influence of potential interfering substances on the determination of captopril were studied. The method was satisfactorily applied to the determination of captopril in commercial captopril tablets and urine samples. In the second part of this project, oxidation of benserazide and levodopa have been studied at modified carbon paste electrode with chloranil as a heterogeneous mediator, using electrochemical impedance spectroscopy, cyclic and linear sweep voltammetry, and chronoamperometry as diagnostic and measurement techniques. Cyclic voltammetric measurements showed that, the catalytic current of the system depends on the concentration of the drugs. Under the optimized conditions, the calibration curves were linear in the concentration range of 3.0×10 -5 – 1.0×10 -4 M and 1.0×10 -4 – 3.2×10 -3 M for benserazide, with a LOD 5.9×10 -6 M and 5.0×10 -5 – 1.0×10 -4 M and 1.0×10 -4 – 5.0×10 -3 M for levodopa with a LOD of 4.7×10 -6 M. The relative standard deviation for 5.0×10 -5 and 2.0×10 -4 M benserazide and 5.0×10 -4 M levodopa were 2.0, 1.5 and 2.0%, respectively. The chronoamperometry was used to determine the catalytic rate constant and diffusion coefficient for the catalytic reaction of chloranil with benserazide. The proposed method was used for the determination of benserazide and levodopa in human urine.