Central type benzodiazepine (BDZ) receptor binding in spontaneously epileptic rats (SER) and their parent strains, tremor rats and zitter rats, and Kyoto/Wistar rats were investigated. Significantly lower BDZ receptor densities (Bmax) and no differences in affinity (KD) were found in the hippocampus of the two epileptic strains, SER and tremor rats, in comparison with Kyoto/Wistar rats and zitter rats. This abnormality is considered to be due to a tremor gene and to be related to absence-like seizures in SER and tremor rats. A significant decrease of KD and an increase of Bmax in the brain stem were found in SER in comparison with Kyoto/Wistar rats. These changes may be due to a zitter gene, since zitter rats show the same tendency, and they may be related to tonic seizures in SER. Bmax was significantly increased in the cerebellum and hippocampus of the zitter rats, while KD was not changed, in comparison with Kyoto/Wistar rats and tremor rats. These changes may reflect the relatively selective loss of tissue lacking BDZ receptors or an upregulation in response to the loss of GABAergic neurons in zitter rats.