Context. Endometriosis is defined as the presence of a functional endometrial layer with endometrial glands and stroma outside the uterine cavity which induces a chronic inflammatory reaction and is also linked to pelvic pain and infertility. Metformin has a modulatory effect on ovarian steroid production and has anti-inflammatory properties. It was shown that in vitro metformin suppresses inflammatory response, activation of aromatase and proliferation in endometriotic stromal cells. Objective. This study was planned to determine the effect of metformin on endometriotic implants in rats. Methods. Prospective randomized controlled experimental trial. Participants: Twenty female, Wistar albino rats. Intervention: Endometriosis was induced by autotransplanting small pieces of uterus on peritoneum of anterior abdominal wall that formed vascularized cysts. After 2 weeks, laparoscopy was performed to evaluate the formation of the model and the size of endometrioid implants. After randomization, two groups were formed: the main group (n = 9) and the control group (n = 11). The rats of the main group received metformin orally 300 mg / kg / day. The duration of drug administration was 21 ± 3 days. After three weeks of treatment, the average surface area of endometrioid implants was measured, dynamics of the size changes were assessed and compared between groups. Main Outcome Measure(s): After three weeks of treatment, the average area of endometriotic implants in the main group was significantly lower than in the control group. When metformin was used, complete resorption of the implants was noted in 39% of cases, in other cases the average area of endometrioid lesion decreased by 58 ± 7%. In the control group, the focal size increased by 26 ± 7%. The average area of endometriotic implants in the main group 9.1 ± 2.9 mm2 was significantly (p < 0.05) lower than in the control group 18.3 ± 5.2 mm2. These results were confirmed by histological examination Results: Metformin caused significant regression of endometriotic implants. Conclusions: The present study confirms that metformin is effective in the treatment of endometriosis as measured by a decrease in the size of endometriotic implants.