STUDY QUESTION Is pigment epithelium-derived factor (PEDF) expressed in the rodent endometrium and can it be utilized to treat endometriosis without negatively affecting reproductive parameters?SUMMARY ANSWERPEDF is dynamically expressed in rat endometrium throughout the estrous cycle in a reciprocal manner to vascular endothelial growth factor (VEGF); it possesses potent therapeutic properties for endometriosis that do not compromise the reproductive parameters. WHAT IS KNOWN ALREADY Endometriosis pathogenesis depends mainly on neovascularization, with a high local level of VEGF. PEDF, a 50 kDa secreted glycoprotein with a potent anti-angiogenic activity, negates several strong pro-angiogenic factors, such as VEGF. STUDY DESIGN, SIZE, DURATION Rat endometrial samples were collected at various days of the estrous cycle (n = 5 rats/day) and mRNA of VEGF and PEDF was determined. Endometriosis was induced by transplanting uterine pieces onto the inner surface of the abdominal wall of recipient rats, resulting in proliferation of the endometrial transplants. Recipient rats were randomly injected intravenously (IV), every third day for the next 3 weeks, with either Tris ('control'; n = 7) or recombinant PEDF (rPEDF; 2 mg/kg/day; 'PEDF prevention'; n = 7), while others were IV injected every third day starting from Day 9 after grafting until the end of 3 weeks, with rPEDF (2 mg/kg/day; 'PEDF treatment'; n = 6). The effect of rPEDF on the duration of the estrous cycle and on the number of ovulated oocytes was evaluated in rats that were randomly divided into four groups and were injected with either Tris or rPEDF every third day for 3 weeks: naive rats (n = 6); rPEDF-treated rats (n = 5); endometriosis-induced rats (n = 5); or endometriosis + rPEDF rats (n = 6). MATERIALS, SETTING, METHODS Reproductive parameters: the estrous cycle was evaluated by daily vaginal smears, and the number of ovulated oocytes in the oviductal ampullae of estrus rats was counted. The efficiency of endometriosis induction and treatment was evaluated on the third week after endometrial transplantation, on the day of pro-estrus. Endometrial transplants were isolated and weighted. PEDF and VEGF were monitored by quantitative PCR and immunohistochemistry using confocal microscopy. MAIN RESULTS AND THE ROLE OF CHANCE PEDF mRNA and protein were dynamically expressed in the endometrium all throughout the estrous cycle, reciprocally to VEGF; VEGF was highly expressed during estrus while PEDF expression was low, and vice versa at metestrus II. The weight of the endometrial transplants was significantly reduced after PEDF administration (13% of control for 'PEDF treatment' rats; 7% of control for 'PEDF prevention' rats; P < 0.001). Histology of the transplants' remnants showed a complete loss of their endometrial characteristics. Furthermore, the level of VEGF mRNA in the transplants of PEDF-administered rats was significantly lower (P < 0.05) than in transplants of control rats. Administration of rPEDF had no effect on the estrous cycle or ovulation rate of naive rats, while it had a significantly beneficial effect on the low ovulation rate of endometriosis-induced rats (P < 0.05). LIMITATIONS, REASONS FOR CAUTION The experiments were performed in a rat model. WIDER IMPLICATIONS OF THE FINDINGS The endometriosis therapeutic potency of PEDF that is exerted reciprocally to VEGF and does not compromise reproductive parameters offers a rational for using PEDF as a treatment for endometriosis with a potential of treating other reproductive angiogenic-related pathologies. STUDY FUNDING/COMPETING INTEREST(S) This work was partially supported by a grant from the Lau-Mintz Foundation (Tel-Aviv University) to R.S. The authors have no competing interests to declare.