Tumor necrosis factor a (TNF-a) Is a cytokine that Is responsible, In part, for several aspects of the acute-phase response to Ic the genea-tion of fever. TNF-a has direct effects on ceal nerous system neurons deep within the yp -MUS that are Invlve In producing the febrile response, but the blood-brain harrier prevents circulating TNF-a fom having access to these sites. We therefore have hypodtized that TNF-a my be Frwodnced in the brain and used as a In the cerebral of the acute-phase response. We used in situ h to determine the ditbution of production of TNF-a mRNA in the mouse brain after systmaic of lpopolysac- charide. During the initial phase of fever, hybridization was observed in perisar cells and neurons in cicmventrlcu-lar organs, d cuar of the lamina termi-