The stability of methadone hydrochloride in 0.9% sodium chloride injection in flexible polyvinyl chloride containers was studied. Commercially available methadone hydrochloride 20 mg/mL and 25-mL single-dose bags of 0.9% sodium chloride injection were used. Six samples each were prepared at methadone hydrochloride concentrations of 1, 2, and 5 mg/mL. The solutions were stored at room temperature and were not protected from light. Immediately after preparation and after two, three, and four weeks of storage, each of the 18 samples was divided into three aliquots, each of which was analyzed in duplicate for methadone hydrochloride concentration by gas chromatography. There was less than 10% change in methadone hydrochloride concentration in any sample throughout the four-week study period. Methadone hydrochloride at concentrations of 1, 2, and 5 mg/mL prepared in commercially available flexible polyvinyl chloride containers of 0.9% sodium chloride injection and stored at room temperature without deliberate protection from light is stable for at least four weeks.