The stability of papaverine hydrochloride and phentolamine mesylate combined in a single vial was studied. Injectable mixtures (10 mL) of papaverine hydrochloride 300 mg and phentolamine mesylate 5 mg (from two sources) were prepared by adding the contents of one vial of lyophilized phentolamine mesylate to the contents of one vial of papaverine hydrochloride injection. The vials were stored at 5 degrees C and 25 degrees C. Duplicate aliquots of the mixtures were obtained, and the concentrations of papaverine hydrochloride and phentolamine mesylate remaining at time 0 and after 1, 2, 5, 10, 20, and 30 days were determined in triplicate by a stability-indicating high-performance liquid chromatographic assay. The concentration of papaverine hydrochloride stored in the vials remained constant (less than 1% loss) over the 30-day period at both 5 degrees C and 25 degrees C. Phentolamine mesylate was less stable than papaverine but still retained more than 97% of its original concentration after 30 days at 5 degrees C and more than 95% of its original concentration at 25 degrees C. Papaverine hydrochloride and phentolamine mesylate are stable in injectable mixtures when stored for up to 30 days at 5 degrees C or 25 degrees C.