We examined the anticonvulsant effects of BW1003C87 (5-(2,3,5-trichlorophenyl)-2,4-diaminopyrimidine ethane sulphonic acid), which is structurally related to the new antiepileptic drug, lamotrigine, and compared its effects to those of the conventional antiepileptic drugs, phenytoin and carbamazopine, using the rat amygdala-kindling model of epilepsy. BW1003C87 (2.5-10 mg/kg, i.p.) had potent and long-lasting (48 h after single administration) effects on amygdala-kindled seizures. The effects of BW1003C87 were completely reversed when the stimulus intensity was increased to 2 or 3 times the threshold determined. Since the same effects on seizure threshold were obtained for phenytoin and carbamazepine in the present study and for lamotrigine in our previous study, we propose that the principal mechanism of these antiepileptic drugs, which act primarily on voltage-sensitive Na+ channels, is significant elevation of the seizure threshold in epileptogenic foci and that BW1003C87 has a profile similar to that of these drugs.