Hyperthermia was induced in mature rats by immersing them in 20 cm of 45 degrees C water for 4 min. Rats were made hyperthermic once every 4 days for six exposures. A significant number had at least one convulsion by the third exposure. During the seventh exposure, 2 weeks after the sixth exposure, twice as many rats experienced convulsions. Two weeks later, four rats that had convulsed following exposure to heated water at an earlier time convulsed on exposure to a strobe light flashing at 25 Hz. The percentage of rats having experienced at least one convulsion increased dramatically when tested 2 and then an additional 3 months later. Spontaneous (handling-induced) convulsions also occurred in a few rats that had been exposed to heated water previously. The data indicate that repeated exposure to this type of hyperthermia can result in an increase in convulsive susceptibility in mature rats and may be a useful, noninvasive model for studying kindling, febrile convulsions, and epilepsy in rodents.