This study was designed to evaluate the penicillin-induced epilepsy model in terms of dose-response relationship of penicillin used to induce epilepsy seizure on hippocampal neuron number and hippocampal volume in Sprague-Dawley rats. Seizures were induced with 300, 500, 1500 and 2000 IU of penicillin-G injected intracortically in rats divided in four experimental groups, respectively. Control group was injected intracortically with saline. Animals were decapitated on day 7 of treatment and brains were removed. The total neuron number of pyramidal cell layer from rat hippocampus was estimated using the optical fractionator method. The volume of same hippocampal areas was estimated using the Cavalieri method. Dose-dependent decrease in hippocampal neuron number was observed in three experimental groups (300, 500 and 1500 IU of penicillin-G), and the effects were statistically significant when compared to the control group (P < 0.009). Dose-dependent decrease in hippocampal volume, on the other hand, was observed in all three of these groups; however, the difference compared to the control group was only statistically significant in 1500 IU of penicillin-G injected group (P < 0.009). At the dose of 2000 IU penicillin-G. all animals died due to status seizures. These results suggest that the appropriate dose of penicillin has to be selected for a given experimental epilepsy study in order to demonstrate the relevant epileptic seizure and its effects. Intracortical 1500 IU penicillin-induced epilepsy model may be a good choice to practice studies that investigate neuroprotective mechanisms of the anti-epileptic drugs. (c) 2008 Elsevier Inc. All rights reserved.