To determine whether the p44/p42 mitogen-activated protein (MAP) kinase [extracellular signal-regulated kinases 1/2 (ERK1/2)] signaling pathway is involved in the activation of corticotrophin-releasing hormone (CRH)-containing neurons in the hypothalamic paraventricular nucleus (PVN) following bacterial lipopolysaccharide (LPS) administration, Sprague-Dawley rats were injected with LPS and studied after 2, 6, 9 and 12 h. In saline treated controls, isolated, weak phospho-ERK1/2 immunoreactive neurons were observed in the PVN. However, a dramatic increase in phospho-ERK1/2 immunoreactivity was apparent in the PVN within 2 h of LPS administration, and gradually declined to baseline levels 9–12 h following injection. By double-labeling immunofluorescence, all CRH-containing neurons in the PVN contained phospho-ERK1/2 2h after LPS. Intracerebroventricular administration of the MAP kinase inhibitor, PD98059, prevented LPS-induced ERK1/2 phosphorylation, cFos activation, and the increase of CRH gene expression in the PVN but had no effect on cFos activation in brainstem A1/A2-C1/C2 regions. We conclude that LPS rapidly increases the phosphorylation of ERK1/2 in CRH-containing neurons in the PVN and that activation of MAP kinases is necessary for the LPS-induced activation of the hypothalamus-pituitary-adrenal (HPA) axis.