Anticonvulsant action of MK-801, a novel non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors, was investigated in the kindling model of epilepsy in rats. The results obtained were as follows. (1) Both the seizure stage and afterdischarge duration of previously kindled seizures from the amygdala were significantly suppressed following systemic injection of MK-801 (0.25-4 mg/kg) in a dose-dependent manner. The maximum effects were observed between 2 and 4 after the injection. (2) The MK-801 also showed significant anticonvulsant effects on kindled seizures from the frontal cortex and the ventral and dorsal hippocampus. The efficacy, however, significantly differed between these kindled sites. (3) Daily treatment of MK-801 (0.25 and 1 mg/kg) prior to each electrical stimulation of the amygdala significantly retarded kindling seizure development and increased the total amount of afterdischarge (accumulated AD) required to reach the first stage 5 seizure. During drug sessions of 1 mg/kg MK-801 for 19 days, all rats showed only partial seizures and the growth of afterdischarge was strongly prevented. (4) Pretreatment with reserpine did not antagonize the anticonvulsant effects of MK-801 on previously kindled seizures from the amygdala, suggesting that the effects may not be mediated by catecholaminergic systems. These results indicate that MK-801 has potent anticonvulsant actions on kindled seizures from both limbic and cortical foci, the NMDA system may play a critical role in the seizure-triggering mechanism of kindling. The possible application of NMDA antagonists in clinical epilepsy is suggested.