This article describes a high-performance liquid chromatography (HPLC) method for the simultaneous determination of morphine (M) and its principal metabolites morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G), and normorphine (NM) in plasma. All four compounds are extracted from plasma using a C8 solid-phase extraction column, separated by reverse-phase HPLC on a C18 analytical column, and detected by spectrofluorometry at 210 nm excitation wavelength. The method takes advantage of the compounds' native fluorescence, so that derivitization is not required. Samples have been quantified over a concentration range of 25-100 ng/ml M and NM, 50-200 ng/ml M3G, and 100-300 ng/ml M6G, using nalorphine (500 ng/ml) as internal standard. Within-run and between-run errors were less than 10% for morphine and less than 13% for all the metabolites. The lower limit of quantitation for morphine is 10 ng/ml. The accuracy of the method was confirmed by including quality controls fitted to the standard curves of each compound. The assay described in this article represents a simplification of previous versions of the method, which included cumbersome extraction procedures and multiple detectors. For the first time, an internal standard has been employed. The assay is reliable and easy to use and can be performed in any therapeutic drug monitoring laboratory.