1. Lipoxin (LX) A(4) and aspirin-triggered-LX (ATL) are endogenous lipid-derived mediators that regulate leukocyte trafficking via specific LXA(4) receptors (ALX), and are involved in endogenous anti-inflammation and resolution. Both LXA(4) and ATL are produced by rat tissues in vitro as well as in vivo. In rats, LXA(4) and ATL exhibit potent physiological and pathophysiological roles. Thus, we set out to determine whether ALX is expressed in rat tissues and its potential role in modulating leukocyte trafficking with LXA(4) and ATL. 2. In rats, a stable analog of ATL, when given intravenously with two consecutive doses at approximately 60 microg kg(-1) each injection, significantly inhibited neutrophil infiltration (approximately 43%) and protein extravasation (approximately 42%) in a casein-induced peritonitis. 3. The rat orthologue of ALX was cloned from peripheral blood leukocytes encoding a putative G protein-coupled receptor (GPCR). It gave approximately 74 and approximately 84% homology, respectively to the deduced amino-acid sequences of the human and mouse ALX. 4. Tissue distribution analysis by RNase protection revealed that this rat receptor is expressed in tissues/cells, where LXA(4) displays physiological and pathophysiological roles, namely, lung, kidney and leukocytes. 5. The rat orthologue of ALX gave specific radioligand binding with [(3)H]LXA(4) and [(125)-Tyr]-annexin 1-derived peptide with apparent K(d) values of 5 and 820 nM, respectively, that are at levels comparable to those of the human ALX. 6. Activation of rat ALX inhibited tumor necrosis factor alpha-mediated nuclear factor kappaB activity in a ligand-dependent manner utilizing a luciferase reporter gene system. 7. Together, these results are the first demonstration of a rat ALX that is conserved in both structure and function suggesting that ALX plays key roles in regulating effector immune responses from murine to human species.