OBJECTIVE: This study was attempted to look at the effect of dexamethasone on the luteolysis of corpus luteum in rats by immunohistochemical study. METHODS: Counting with an optical microscope was conducted to make a comparison on difference in luteolysis and penetration of macrophage into three groups: control group of 30 female rats at 8 weeks of age, dexamethasone 0.1 mg administered group, and dexamethasone 1mg administered group. RESULTS: As a result of TUNEL immunostaining, the percentage of luteolysis was significantly reduced in both dexamethasone 0.1 mg administered group and 1 mg administered group, and after ED1 immunostaining, macrophage invasion was reduced in dexamethasone 1 mg administered group. As a consequence of ED1 immunostaining, the immune response of macrophage was much decreased in dexamethasone 1 mg administered group than control group. CONCLUSION: Dexamethasone works on luteal cell, so it can suppress apoptosis. It can suppress luteolysis by suppression macrophage invasion into corpus luteum or suppress macrophage activation in corpus luteum.