The long-term compatibility of sodium valproate (VPA) and pyridoxal phosphate calcium (PAL-P) was examined. Mixtures of VPA and PAL-P in heat-sealed packages (polyethylenelaminated glassine paper) were stored under various conditions for 90 days. Under the most severe condition (30°C and 90% relative humidity), the color of the mixture changed to dark yellow and the weight increased by 20-35%. Although the VPA content in the mixture did not change, the PAL-P content decreased by approximately 15%. When the mixture was stored in the paper bag at 20-30°C and in the air-tight receptacle at 25°C and 50% relative humidity, the decrease in the VPA or PAL-P content was less than 7%. Therefore, when preserved over a long-term period, the mixture of these medicines should be avoided in high-temperature and high-humidity condition.