Buprenorphine is a partial μ-opioid agonist used for analgesia. Due to the small size of laboratory rodents, buprenorphine HCl is typically diluted 10- or 20-fold with a sterile diluent, such as saline, for accurate dosing. Protocols for preparing and storing diluted buprenorphine vary by institution, and little published information is available regarding stability and beyond-use dating of specific buprenorphine preparations. The purpose of this study was to determine the chemical and microbiologic stability of diluted buprenorphine stored for a maximum of 180 d. Buprenorphine HCl was diluted 1:10 into sterile bacteriostatic saline by using aseptic technique. Diluted samples were stored in glass vials or plastic syringes, protected from light, and maintained at refrigerated or room temperature for as long as 180 d. Aerobic and anaerobic cultures on all stored samples were negative for bacterial and fungal growth. According to HPLC analysis, diluted buprenorphine stored in glass vials experienced less than 10% loss when stored for 180 d at either refrigerated or room temperature. However, the concentration of buprenorphine stored in syringes declined rapidly to more than 80% loss at room temperature and 28% loss in the refrigerator after 180 d. According to the results of this study, diluted buprenorphine stored in glass vials retains more than 90% of the initial concentration and is microbiologically stable for 180 d. However, our data suggest that, regardless of the duration, storing diluted buprenorphine in plastic syringes is inadvisable.