The stability of fentanyl citrate diluted with 0.9% sodium chloride injection for use in portable infusion pumps was studied. The commercially available injection containing 50 micrograms of fentanyl per milliliter was diluted to a concentration of 20 micrograms/mL. Twelve 100-mL portions of the dilute solution were placed in polyvinyl chloride infusion pump drug reservoirs; six were stored at 3 degrees C and six at 23 degrees C; three at each temperature were overwrapped with polypropylene-Mylar. Initially and after 5, 10, 20, and 30 days of storage, 1-mL samples were taken from each reservoir, inspected for color change and precipitation, and assayed for fentanyl concentration by high-performance liquid chromatography. Initially and on day 30, pH of the samples was checked. No precipitation or change in color or pH was observed. No substantial decrease in fentanyl concentration was found in either the wrapped or unwrapped samples at either temperature, although concentrations on day 30 in the samples at 23 degrees C were slightly lower than those at 3 degrees C. Under the conditions studied, fentanyl citrate solutions containing 20 micrograms of fentanyl per milliliter can be stored for 30 days in polyvinyl chloride reservoirs for portable infusion pumps.