The chemical stability of naltrexone hydrochloride injection was studied by using a stability-indicating high-performance liquid chromatographic assay method that was developed in our laboratory. The concentrations of the drug were related directly to peak heights, and the percent relative standard deviation based on five injections was 1.0. The products of decomposition and excipients present in the dosage form did not interfere with the developed assay method. The injection was stable for at least 42 days when stored in clear glass vials at room temperature. The pH value of the injection decreased from 6.4 to 5.5 after 42 days of storage; the physical appearance of the injection did not change during the study period.