The possible role played by superoxide dismutase (SOD), a major defense system for counteracting the toxic effects of oxygen free radicals, in amygdaloid (AM) kindling was examined in rats. A significant increase of total SOD activity in the whole brain was observed 30 days after completion of AM kindling. Intra-AM injection of 3 ng of one of the 2 SOD enzymes present in mammalian brain, i.e. cytosolic SOD containing copper and zinc (CuZn-SOD) caused suppression of kindled seizure. These results suggest that SOD participates in the persistence of AM kindled seizure susceptibility and the initiation of kindled AM seizure.