Abstract: Objective To investigate the effects of Xiaochaihu Tang (XCHT), a traditional Chinese medicinal preparation, on ectopic endometrium in rats.Methods In rat models of endometriosis, the effect of XCHT of ultrastructure of the ectopic endometrium was observed by transmission electron microscopy.Results After treatment with XCHT at moderate (500 g/L) or high dose (750 g/L), ectopic endometrial growth was significantly inhibited (P<0.01); small-dose XCHT (250 g/L) was less effective (P<0.05). The volume of endometrium implant in the control group remained unchanged (P>0.05). The glandular cells in the endometrial implant after therapy with moderate- or high-dose XCHT showed characteristic features of apoptosis, presented by decreased cell size, karyopyknosis, cytoplasm and nuclear chromatin condensa- tion, increased density and presence of apoptotic bodies. Some stromal cells displayed degenerative and necrotic changes.Conclusion XCHT may inhibit the growth of ectopic endometrium by inducing cell death in the form of apoptosis and necrosis.