We and others have reported that c-fos protein is induced in the hypothalamus and brain stem of the rat following central and peripheral injections of endotoxin (lipopolysaccharide; LPS). We have now examined possible mechanisms through which LPS induces c-fos protein. The cyclooxygenase inhibitor indomethacin and the glutamate NMDA antagonist MK801 inhibited c-fos protein in the paraventricular nucleus (PVN), supraoptic nucleus (SON), and the A1/A2 regions of the brain stem induced by IP or IV injections of LPS (40 micrograms). The H1 histamine antagonist diphenhydramine, but not the H2 histamine antagonist cimetidine, reduced the amount of c-fos labeling. MK801 also attenuated the effects of stress (foot shock) on c-fos protein; however, indomethacin had no effect on c-fos protein induced by stress. We next examined the importance of visceral afferent innervation on the response to LPS or stress. Subdiaphragmatic vagotomy completely blocked the induction of c-fos protein following IP injections of LPS; however, vagotomy had a minimal effect on c-fos protein induced in the PVN and SON following IV injections of LPS, but potentiated c-fos induction following foot shock. Thus, prostaglandin synthesis, glutamate release, histamine receptors, and visceral afferents represent functional biochemical and neural pathways through which endotoxin activates c-fos protein in specific autonomic and neuroendocrine regulatory nuclei. Activation of NMDA glutamate receptors may represent a final common pathway for the induction of c-fos protein in the brain induced by both endotoxin and stress.