Objective Long acting luteinizing hormone-releasing hormone (LHRH) antagonists designed to be protease resistant consisted of a series of novel decapeptides structurally similar to LHRH.The aim of this study was to evaluate the in vitro metabolic stability of the LHRH decapeptides using the pancreatin and homogenates models and identify the metabolites in rat liver homogenate for the purpose of illustrating the metabolism features of decapeptides.Methods The pancreatin model and cassette dosing (N in one) method were used to evaluate stability of 12 decapeptides, then five more stable decapeptides were chosen to study further by rat liver,kidney and lung homogenate model.The major metabolites were identified by LC-ES1-MS/MS.Results The half lives of 11 decapeptides were from 44 min to 330 min in pancreatin model.The half lives of the five decapeptides in rat liver, kidney and lung homogenates were between 8 min and 462 rain.Among five decapeptides, the half life of LY618 in liver homogenate was the shortest with 7.5 min.The most stable decapeptides were LY616 and LY608 with the half lives of 36 minutes in liver homogenate.Two major cleavage sites were found by analysing the metabolites ofLY618 peptide in rat liver homogenate, the sites between the Pal3-Ser4 and the Leu7-Ilys8 peptide bonds.The major metabolites were produced via cleavages ofpeptide bonds at these sites, and further metabolic reactions such as hydroxylation, oxidative dechlorination, alcohol dehydration and isopropyl dealkylation were also observed.Conclusion LY616 and LY608 peptides are the most stable decapeptides in the tested peptides.To identify the influence of these organs on enzymatic degradation, homogenates of liver or kidney can be used for enzymatic stability studies.A further in vivo study of these peptides is needed to confirm these in vitro results.