The psychotherapeutic drug chlorprothixene is shown to adsorb strongly onto a glassy carbon surface in an open circuit. By using this phenomenon to preconcentrate the drug at a glassy carbon electrode prior to differential-pulse voltammetric measurements, sensitivity at the ppb level is readily achieved. The adsorptive stripping response was evaluated with respect to electrolyte, solution pH, accumulation time, concentration dependence and other variables. A linear peak current-concentration relationship was observed up to 1 microgram ml-1 of chlorprothixene; the relative standard deviation (at the 0.6 microgram ml-1 level) is 3.2%. For a preconcentration time of 10 min, the detection limit was found to be 2 ng ml-1. The open circuit preconcentration/medium exchange/voltammetric scheme was used to eliminate interference from sample solutions. The application of the method to human urine samples is described.