Background: Combination antiemetic therapy has become a common practice for the prevention of postoperative nausea and vomiting (PONY). The aim of the present study was to evaluate the stability and compatibility of ramosetron hydrochloride and midazolam in 0.9% sodium chloride injection when stored at 4 degrees C and 25 degrees C for up to 14 days. Methods: Admixtures were assessed initially and for 14 days after preparation in polyolefin bags and glass bottles using 0.9% sodium chloride injection as the diluent and stored at 4 degrees C or 25 degrees C. The initial concentrations were 0.3 mg/100 mL ramosetron hydrochloride and 0.5 mg/100 mL midazolam hydrochloride. For all samples, the compatibility parameters (including precipitation, cloudiness, discoloration and pH values) were evaluated. Chemical stability was also determined using high-performance liquid chromatography (HPLC) analysis. Results: After a 14-day period of storage at 4 degrees C or 25 degrees C, the percent of the initial concentration of ramosetron hydrochloride and midazolam hydrochloride in the various solutions were maintained at a minimum of 97%. All of the mixtures remained clear and colourless throughout the observation period, and no colour change or precipitation was observed. Conclusion: The results indicate that admixtures of 0.3 mg/100 mL ramosetron hydrochloride and 0.5 mg/100 mL midazolam hydrochloride in normal saline were stable for 14 days at 4 degrees C or 25 degrees C when packaged in polyolefin bags or glass bottles and protected from light.