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Compatibility of sodium valproate (VPA) fine granules with 48 drugs was investigated by sensory test, and VPA residual potency (%) was measured by nonaqueous titration and gas chromatography. The data on change in compatibility with time were analyzed by scoring and were subject to a time series analysis. Change in compatibility was observed in 23 of the 48 drugs under the severe conditions (30℃, RH 92%),and in 12 drugs under the medium conditions (20℃, RH 75%)・In gas chromatography, when combined with ascorbic acid, residual potency (%) of VPA began to decrease sharply on the first day, thus suggesting that such combination be avoided. There is not always correlation between change in appearance and the results of nonaqueous titration and gas chromatography. Since VPA may be administered for as long as 30 days, caution should be exercised in mixing this drug with other agents.