This study was performed to investigate the influence of the dosing route on chronopharmacological aspect of valproic acid (VPA) in mice, comparing the oral and rectal route. ICR male mice, housed under a standardized light-dark cycle (lights on from 0700 to 1900), were orally or rectally administered 400 mg/kg VPA each at the following scheduled time: 0900, 1300, 1700, 2100, 0100 and 0500. VPA concentrations in plasma and brain were determined by gas-liquid chromatography. There was a circadian rhythm in the electroshock seizure (ES) threshold 30 min after oral VPA administration, with the highest value at the midlight (1300) and the lowest at the middark (0100) (p < 0.01). A significant circadian rhythm was also found in plasma and brain VPA concentrations 30 min after oral administration (p < 0.01). This finding is related to the rhythm in the ES threshold. In contrast to oral administration, no circadian rhythm in the ES threshold, plasma and brain VPA concentrations was observed after rectal administration. These values after rectal dosing showed higher levels in comparison to those after oral dosing. Thus, the rectal route for VPA might have merit to eliminate the time-dependent changes in VPA pharmacologic action and kinetics. The timing of drug administration is an important factor that must be carefully controlled in drug pharmacokinetic and pharmacodynamic studies and must be considered in planning dosing routes.