A sensitive electrochemical method for the determination of four drugs: perphenazine, fluphenazine chlorpromazine and trifluperazine is reported. These compounds exhibited accumulation at carbon paste electrodes in an open circuit. Differential pulse voltammetric determination gave linear calibration curves in the range of 0.02-1 microgram/ml. Detection limits found were 5-7 ng/ml. The drugs in tablets and body fluids were determined directly without separation. The preconcentration mechanism and determination conditions in a new kind of carbon paste piston electrode were investigated. Adsorption and extraction played important roles in accumulation process and the ratio of these two parts was measured by chronocoulometry.