We have investigated the effects of glucocorticoid manipulation on orexin-A-induced feeding and on prepro-orexin mRNA levels in the lateral hypothalamic area (LHA) of the rat brain. Adrenalectomy (ADX) reduced orexin-A-induced feeding over 4 h by about 60% compared with shams, an effect which was reversed by corticosterone (CORT) replacement. ADX had no effect on prepro-orexin mRNA levels in the LHA in either the morning or the evening however, message was upregulated by CORT in the morning but not in the evening. An increased number of emulsion grains per cell in the LHA suggests that this is a specific increase in prepro-orexin mRNA and is not due to an increased number of cells expressing message. Prepro-orexin mRNA levels in the LHA were elevated 4 h following injection of lipopolysaccharide compared with saline-injected controls. Partial but not complete abolition of orexin-A-induced feeding by ADX suggests that orexin-A-induced feeding may be mediated through glucocorticoid-dependent and glucocorticoid-independent pathways. In the morning increased prepro-orexin mRNA following CORT replacement demonstrates that orexin expression is sensitive to increased concentrations of glucocorticoids. However, the lack of effect of ADX on prepro-orexin mRNA levels suggests that endogenous glucocorticoids are not involved in tonic regulation of basal prepro-orexin expression. Overall our data constitute a body of evidence for an integrated relationship between central orexin expression, stress, glucocorticoid manipulation and feeding patterns in the rat.