Concentrations of the polyamines, putrescine, spermidine and spermine were investigated in rat brains, in which chemical kindling or single convulsion had been induced by intraperitoneal injection of pentylenetetrazol (PTZ). A single injection of 60 mg/kg of PTZ produced tonic-clonic convulsion and increased the putrescine concentration 8 h after the injection. At lower doses of PTZ (10 and 30 mg/kg), neither marked behavioral seizure nor significant change in any polyamine concentration was observed. On the other hand, repeated injections of 30 mg/kg of PTZ eventually resulted in intense motor seizures (PTZ kindling) and increased the concentrations of all three polyamines. The most marked increase was detected in putrescine 1-48 h after the intense seizures. The increase in putrescine was clearly higher in PTZ kindling than in single convulsion. These results suggest that increases in polyamine concentrations are involved in neuronal excitability in the epileptic brain.