Annexin 1 (ANXA1) is an important mediator of the regulatory effects of glucocorticoids (GCs) within the neuroendocrine and host defence systems. Recent data suggest that it acts via a formyl peptide receptor (FPR) as a mediator of the negative feedback effects of GCs on adrenocorticotrophic hormone (ACTH) release. In the present study, in vitro and in vivo methods were used to detect and explore further the function of these receptors within the hypothalamo-pituitary-adrenocortical (HPA) axis. Lipopolysaccharide (LPS), given intra-peritoneally (i.p.) or centrally (i.c.v.), increased expression of mRNAs for ANXA1 and Fpr1, Fpr2 and Fpr3 in the spleen, pituitary and adrenal gland, but not in the brain. Given i.p., it also caused inflammatory cell infiltration in the adrenal gland, but not the pituitary, together with decreased vacuolation in the steroidogenic cells, increases in serum pro-inflammatory cytokines and corticosterone (CORT) and a subsequent loss of sensitivity to ACTH. The increases in ANXA1, Fpr1 and Fpr3 expression were dependent on inflammatory cell infiltration (predominantly eosinophils) but those of Fpr2 were not. The decrease in vacuolation was also independent of the inflammatory cell infiltration but was severely compromised by deletion of the genes encoding ANXA1 and Fpr2. Pharmacological studies on isolated adrenal cells in vitro suggest that ANXA1 may act via Fpr2 to inhibit ACTH-stimulated CORT release but that Fpr1 effects a tonic stimulatory effect on ACTH-driven steroidogenesis. These data suggest that ANXA1 and the FPRs play an important role within the adrenal in mediating the HPA responses to endotoxin and provide new evidence to suggest that infiltrating leukocytes are are also important in this regard.