OBJECTIVE: Age, body weight and dose have been shown as important influencing factors for sodium valproate plasma concentrations. However it is unclear whether there is interaction among them and whether the interaction could influence sodium valproate plasma concentrations. This study aimed to evaluate the influence of age, body weight and dose on plasma concentrations of sodium valproate and the interaction among them.METHODS: One hundred and thirty-two children with epilepsy (age: 4 months-6 years, weight: 5-25 kg) were enrolled. Sodium valproate was administered at the dosage of 10-30 mg/kg.d. Plasma concentrations of sodium valproate were measured by high-performance liquid chromatography 3-5 days after administration. The relationship of sodium valproate plasma concentrations with age, body weight, and dose of sodium valproate was examined using variance analysis, pearson correlation analysis and stepwise regression analysis. RESULTS: Age (F=8.630, P<0.01), body weight (F=3.650, P<0.05) and dose of sodium valproate (F=11.720, P<0.01) were influencing factors for sodium valproate plasma concentrations. The interaction between age and oral dose (F=2.484, P<0.05) and the interaction of age and body weight with oral dose (F=4.923, P<0.01) had significant effects on sodium valproate plasma concentrations. Stepwise regression analysis showed that dose of sodium valproate and body weight were entered to the regression equation.CONCLUSIONS: Age, body weight and dose of sodium valproate as well as the interactions between age and dose and between age, body weight and dose were influencing factors for valproate plasma concentrations.