Studies were undertaken to compare tetanus toxin-induced electrographic and behavioral seizure activity in immature and adult rats. Multifocal epileptiform activity was induced by unilateral hippocampal injection of tetanus toxin in 10-day-old immature and in 90-120-day-old adult rats. Bilateral cortical surface electrodes and, in some animals, hippocampal depth electrodes were placed and serial video EEG was recorded for 2 h during the week following tetanus toxin injection. Tetanus toxin-induced interictal spike frequency was significantly higher and seizure duration was significantly longer in immature compared to adult rats. Immature rats tended to have more frequent seizures compared with adults, but the difference was not statistically significant. Ictal behaviors were different for the two groups with immature rats commonly demonstrating wild running seizures which were not seen in adult rats. These findings suggest that there are age-dependent differences in electrographic and behavioral seizure expression in the tetanus toxin seizure model and that the epileptic syndrome in immature rats is more severe.