Objective To establish an useful animal model for research on diagnosis, treatment and drug development for endometriosis. MethodsFemale virginal rats with sexual maturation were induced by estrogen prior to operation. During the laparotomy, a part of right uterus was taken and the endometrium was implanted to the left abdominal wall. A cyst was formed and it was taken out at 16 weeks after implantation, examined by histology and histochemistry. ResultsThe ectopic endometrium growing in the abdominal wall formed a cystic mass with a large cavity containing much mucus. It showed an endometrial structure similar to that of normal endometrium. Glycogen and RNA were observed in the ectopic endometrium. ConclusionThe ectopic endometrium established by this method forming a cystic mass, is palpable as early as at one week after implantation. It is a reliable and convenient animal model for research on endometriosis diagnosis and treatment, and pharmacological and pharmaceutical studies.