The pharmaceutical properties of four vehicles (potato starch, AS; fine lactose, SL; the mixture of the same amounts of AS and SL, AL; extra fine crystalline lactose, EFC) on their mixing ability with antiepileptic drugs (sodium valproate, Depaken®; clonazepam, Rivotoril®; carbamazepine, Tegretol®; phenytoin, Aleviatin®), as well as on their physical properties and operativeness were examined. Angle of repose of AL, AS, SL and EFC was 53°, 45°, 47° and 44°, respectively. The escapabilities of the four vehicles were 30% or less. The required dividing time for EFC, AL, SL and AS was 53, 96, 217 and 1620 seconds, respectively, and significant differences were also found between each vehicle. In the deviation test, EFC showed a significantly lower variation compared with the other vehicles. In an assay study using HPLC after being mixed with each vehicle, no difference was observed in the contents between clonazepam, carbamazepine and phenytoin, however, EFC, and no others, indicated a lower variation in the mixing content with sodium valproate. There was no difference in the hygroscopicity between each vehicle. These results suggest that EFC is a more efficient vehicle than the other tested vehicles, however, AL, which is used at our hospital, is also considered to be useful as a vehicle.