THE tumour necrosis factor alpha (TNFα) protein is normally absent in the brain. Its production in the nervous tissue during pathological processes is commonly attributed to cells of the macrophage or astroglial lineages. However, an immunoreactivity for TNFα has been observed recently in adult mouse brain after a lesion to the hippocampus. The identification, in the present study, of the cells responsible for this synthesis demonstrates a neuronal localization of the TNFα messenger RNA. We propose that neurone-produced TNFα acts as a modulatory effector in post-traumatic regenerative attempts of the brain.