Abstract: OBJECTIVE To characterize the enzymatic kinetics of repaglinide in rat liver microsome and study the effect of losartan on the metabolism of rat liver microsome. METHODS The metabolism of raglenay by establishing the in vitro incubation system of liver microsome in rats. UPLC method was used to detect the concentration of repaglinide in rat liver microsome with lovastatin as internal standard. The enzyme kinetic parameters of V_max and K_mwas calculated by determining the reduction of repaglinide with GraphPad Prism 5.0 software. Repaglinide(44 μmol·L^-1) was jointly incubated with a series of concentrations of losartan(2.5-50 μmol·L^-1) in a 37℃ water bath, respectively. The reduction of repaglinide in liver microsomes was also measured, and the inhibitory effect of losartan potassium on repaglinide was investigated. RESULTS The optimal incubation time in rat liver microsomes was 40 min, and the optimal protein concentration was 1 mg·mL^-1. The enzyme kinetics parameters of repaglinide were as follows:V_max=47.29 μmol·min^-1·(mg·protein)^-1, K_m=51.41 μmol·L^-1. The IC₅₀ value of the inhibitory effect of losartan potassium on repaglinide liver microsomes in vitro was 17.89 μmol·L^-1. CONCLUSION Losartan has a significant effect on the metabolism of repaglinide in liver microsomes. Drug interaction may occur when the two drugs are combined, and there is a risk of hypoglycemia.