We have examined the expression of the inflammatory cytokine, tumor necrosis factor alpha (TNF alpha) in the mouse brain. Using immunohistochemical methods developed, we found anti-TNF alpha immunoreactivity localized in the basal ganglia and other discrete brain structures. Constitutive immunoreactivity, present in normal, unstimulated brain, was observed in glial and microglial-like cells, but it was predominant in neuronal-like cells. Intravenous administration of lipopolysaccharide (LPS) increased TNF alpha transcript levels detected by RT-PCR in specific brain subregions in which contaminating blood cells were removed. The maximal increase occurred within 2 h of LPS injection; transcripts diminished to near control levels in the next 4 h. Immunocytochemical analysis and single-cell RT-PCR analysis of primary cultures of cortical neuronal cells confirmed expression of TNF alpha in cells that also express neuronal-specific enolase RNA. Addition of LPS or recombinant TNF alpha protein to neuronal cultures enhanced expression of TNF alpha transcripts. Our results indicate that in addition to glial and microglial cells, a well-defined subset of neuronal cells also express TNF alpha constitutively; this expression can be altered by both extrinsic (LPS) and intrinsic (TNF alpha itself) factors.