An analytical procedure for simultaneous determination of codeine and morphine in urine is described. The detection of codeine and morphine is based on liquid-liquid extraction and derivatization to the acetylated compounds. The acetylated codeine and morphine are separated by capillary gas chromatography and identified mass spectrometrically by selected ion monitoring (SIM). Quantitative determination was carried out by SIM using nalorphine as internal standard. Excellent linearity was obtained over a concentration range of 25 to 800 ng/mL. The overall recovery for codeine and morphine in the extraction was found to be 58% and 40%, respectively. The on-column sensitivity for both compounds was 2 ng at a peak-to-noise ratio of 5:1. The derivatives, acetylcodeine, diacetylmorphine, and diacetylnalorphine were stable at room temperature for 72 hr.