The stability of Revex, nalmefene hydrochloride injection, has been studied at several temperatures for periods up to 36 months. The data were obtained using a HPLC method for the potency determination, and for the level of the sole degradation product (2,2'-bisnalmefene). These methods were found to be characterized by excellent precision, linearity, and accuracy over the analyte concentration ranges established. The stability data were found to be interpretable using first-order kinetics, and essentially comparable rate constants were calculated for both the potency loss and the formation of 2,2'-bisnalmefene. Applying the Arrhenius equation to these data, a rate constant of 0.00441 month-1 was deduced for the reactions taking place at 25 degrees C. This low value is consistent with the excellent stability exhibited by the product, and amply justifies its shelf life.