During infection/inflammation bacterial lipopolysaccharide (LPS) activates the immune system and thus enhances the level of circulating cytokines. These circulating cytokines induce adaptive processes within the endocrine system and in particular stimulate the HPA axis to increase the level of anti-inflammatory-acting glucocorticoids in the circulation. We have shown recently that LPS stimulates intrapituitary IL-6 production in folliculostellate cells via specific receptors and the p38a mitogen-activated protein kinase/nuclear factor-kappa B pathway. To test the physiological relevance of these findings, we studied whether LPS could enhance ACTH secretion via paracrine-acting intrapituitary IL-6. Lipopolysaccharide stimulated IL-6 secretion both in monolayer and aggregate mouse pituitary cell cultures, but only in aggregates, ACTH secretion was significantly enhanced by LPS. Other hormones, such as GH or PRL, were less stimulated by LPS. My4, an antibody that blocks the interaction of LPS with the LPS receptor CD14, suppressed both LPS-induced IL-6 and ACTH secretion in aggregate cultures. A neutralizing antibody against mouse IL-6 also inhibited LPS-induced ACTH secretion in aggregates. In mouse pituitary fragments, LPS-induced ACTH secretion was blocked by My4 and IL-6 antibodies, identically to re-aggregate cell cultures. LPS-induced ACTH secretion, mediated by intrapituitary IL-6, may represent a pituitary-specific mechanism that stimulates the HPA axis during infection/inflammation.