The aim of this study was to determine the compatibility and stability of morphine hydrochloride and hyoscine-N-butyl bromide combined in solution at three different concentrations and stored in polypropylene syringes at 4 degrees C and 25 degrees C over a period of 15 days. The doses assayed were 20, 60 and 120 mg/day for morphine hydrochloride and 40, 60 and 80 mg/day for hyoscine-N-butyl bromide. These dose ranges were chosen according to daily practice. At both temperatures, all mixtures can be considered as physically compatible since no evidence of incompatibility-that is precipitation, turbidity, colour change or opacity and gas production-were observed. After 15 days of storage, the percentages of hyoscine-N-butyl bromide remaining in the drug mixtures tested ranged between 96.07% and 92.23%. At the end of the study, the percentages of morphine hydrochloride remaining in the drug mixtures were 100% at both temperatures.