We investigated the effects of single and repeated administrations of S-312-d (methyl-4,7-dihydro-3-isobutyl-6-methyl-4-(3-nitrophenyl)-thieno-[2,3-b]pyridine-5-carboxylate), a newly synthesized L-type Ca²⁺-channel blocker, on tonic convulsions and absence-like seizures in the spontaneously epileptic rat (SER: zi/zi, tm/tm), a genetically based animal model of human epilepsy. Single oral administrations of S-312-d dose-dependently inhibited tonic convulsions and the effects lasted for more than 2 h, although they did not attenuate the absence-like seizures. We also examined the effects of repeated administrations of S-312-d at 1 mg/kg once a day for 4 days on SER. A significant decrease in the number and total duration of tonic convulsions was observed 45 and 75 min after the first administration of the drug, respectively. The effects lasted for 24 h without changes in the background EEG or blood pressure. This inhibitory effect on the tonic convulsions was gradually strengthened by subsequent daily administrations of S-312-d and lasted for 3 days after the cessation of drug treatment. In contrast, the repeated treatment with S-312-d did not influence absence-like seizures of SER. These results suggest that S-312-d is a candidate drug that has antiepileptic effects against the convulsive seizures in human epilepsy.