A brief period of rapidly recurring hippocampal seizures can lead to the progressive development of a permanent increase of seizure susceptibility over several weeks, so-called 'delayed kindling'. We have analyzed seizure parameters critical for the induction of delayed kindling in two strains of rats characterized by fast and slow rates of traditional kindling, respectively. Forty seizures were produced during about 3 h by electrical kindling stimulations every 5 min in the ventral hippocampus. The fast rats displayed several generalized convulsions and had long periods of epileptiform activity, whereas the slow animals only exhibited brief, focal seizures. Changes in excitability were determined after 4 weeks using five test stimulations, and 2 weeks later by subjecting all animals to traditional hippocampal kindling. The fast rats showed clearly enhanced responsiveness at these time points, whereas no evidence of permanently increased seizure susceptibility was obtained in the slow rats. Our data indicate that the long-lasting stimulus-evoked seizures are mainly responsible for inducing delayed kindling, whereas the number of seizure events or generalized convulsions, and the total duration of epileptiform activity are less important. We hypothesize that long seizure episodes may be necessary to trigger the cascade of gene changes regulating the development of epilepsy.