A simple quantitative method for determining codeine, norcodeine, and morphine in urine by high-performance liquid chromatography (HPLC) is described. Urine samples are hydrolyzed and extracted through differential pH extraction. The subsequent use of HPLC allowed separation and quantitation on a reverse phase system with ultraviolet detection. A comparison study of the concentrations of single and therapeutic doses of codeine in urine over a period of time showed that morphine, the major metabolite, increases in proportion to codeine, eventually surpasses the codeine level, and remains higher for the duration of time that the drugs are detectable.