Seizure susceptibility in male aged rats (21-25 mo. age) was investigated by pentylenetetrazol (PTZ)-induced seizures and amygdaloid kindling seizures. Male adult rats (3-4 mo. of age) were used as the control group. During 60 min. after the administration of PTZ (70 mg/kg, s.c.), the aged rats had a higher incidence and a significantly longer duration of generalized convulsion than the control group. On the other hand, the kindling rate of the aged rats was significantly slower than that of the control. The aged rats remained longer at stage 1-2 indicative of partial seizures. However, the aged rats skipped stage 3 on their way to stage 4-5 (generalized seizures), and showed a more violent stage 5 seizure than the control group. They had a longer duration of afterdischarges (AD) through the kindling process with a significantly faster propagation of AD to the contralateral amygdala as compared with the control group. The present study suggests that aged rats are prone to convulsion, while they have a difficulty in acquisition of epileptogenesis.