Objective To identify the transformation product S-allyl-mercapto-L-cysteine of allicin in rats after administration,and to investigate the changes of S-allyl-mercapto-L-cysteine in plasma and skin tis-sues.Methods SD rats,half male and half female,with a body weight of(200±20)g,were randomly divided into 8 groups,which were blank group and groups at different time after administration.6 rats are used at each time point,rats in each group were applied to the back of rats at each time point,and blood and skin tissues were taken successively at 0,5,10,15,20,25,30 and 60 min,respectively.UPLC-MS/MS was used to identify S-allyl mercaptol 1-cysteine,the transformation product of allicin,in blood and skin tissues and its changes.Results Allicin was stable in the blank solvent for up to 150 minutes and in deproteinized plasma for up to 8 hours.The reaction between allicin and cysteine produced S-allyl-mercap-to-L-cysteine,with over 90%of the reaction occurring within 30 minutes.The transformation product S-allyl-mercapto-L-cysteine in plasma was stable within 4 hours.UPLC-MS/MS was used to identified and determined S-allyl-mercapto-L-cysteine,the reaction product of allicin with endogenous cysteine,in both skin tissue and plasma samples.The concentrations of S-allyl-mercapto-L-cysteine in plasma and skin gradually increased within 30 minutes after administration of garlic ointment,and decreased after 60 mi-nutes.Conclusion Allicin can rapidly react with endogenous cysteine in plasma and tissues,and this study provides important evidence for subsequent research on the pharmacokinetics of allicin.