A simple method is described for the determination of morphine in biological fluids (plasma and urine) by high-performance liquid chromatography with coulometric detection. The detector consists of three porous electrodes which are arranged in a way to give maximum selectivity and sensitivity. Quantitation of morphine is performed using naloxone as the internal standard. Sample preparation involves an extraction procedure from plasma and urine. The limit of quantification of the method is 1 ng/mL plasma. The methodology was applied to measure blood levels in rats after subcutaneous administration of a slow-release morphine pellet.