The p53 tumor suppressor gene has been implicated in apoptotic cell death. The present study was conducted to investigate whether expression of p53 protein is increased in association with kainic acid-induced neuronal apoptosis. Adult male Sprague-Dawley rats were treated systemically with the glutamate analog kainic acid, and sacrificed either 4 or 30 h after the onset of seizure activity. Immunohistochemistry was performed on paraffin-embedded sections using an anti-p53 polyclonal antibody. At both time points, increased p53 immuno-reactivity was observed predominantly in the nucleus of apoptotic neurons. These findings lend additional support to the hypothesis that p53 is a marker of neuronal apoptosis in the CNS, and suggest that nuclear accumulation of p53 protein may be an important mediator of neuronal death.