The critical susceptibility to seizure in immature hippocampus of rat brain was investigated using the kindling model of epilepsy. Hourly electrical low-intensity stimulations applied to the left dorsal hippocampus of suckling rats induced longer initial AD duration (p less than 0.01) with faster development of kindling seizure than in that of adult rats. In contrast to the marked reduction of wet dog shakes (WDS) in adult rats, in suckling rats persistent and prolonged WDS were observed as kindling progressed. The faster development of hippocampal kindling in suckling rats than in adult rats was explained by the different excitability of the stimulated brain site and the different extent of neural connections in many brain structures, which participate in generalization of seizures arising from a stimulated site.