STUDY QUESTION Does signaling via the cannabinoid (CB 1 ) receptor play a role in the pathogenesis of endometriosis in a mouse model? SUMMARY ANSWER Mice treated with a CB 1 agonist developed larger ectopic lesions, while less severe lesions developed in the absence of functional CB 1 expression. WHAT IS KNOWN ALREADY The expression of components of the endocannabinoid system has been demonstrated in both mouse and human uteri. CB 1 receptors are expressed in human epithelial and stromal cell lines derived from eutopic endometrium and deep infiltrating endometriosis nodules. STUDY DESIGN, SIZE, DURATION This was a randomized study in a mouse model of endometriosis. In a first set of experiments, mice with endometriosis were treated with the CB 1 receptor agonist methanandamide (MET) (5 mg/kg, n = 20) on Days 1-5 and 8-12. In a second set of experiments, endometriosis development was evaluated in CB 1 -/- mice and in their wild-type (WT) littermates. PARTICIPANTS/MATERIALS, SETTING, METHODS Endometriosis-like lesions were induced in Balb/c and C57/Bl6 mice. Two weeks after disease induction, the lesions were counted, measured and either included for immunohistochemistry analysis or frozen for gene expression profiling by semi-quantitative real-time PCR. To limit the role of chance, the experiments were conducted under standardized laboratory conditions with appropriate controls. MAIN RESULTS AND THE ROLE OF CHANCE The lesion total volume was significantly higher in MET-treated compared with vehicle-treated mice (P < 0.05). Expression levels of mRNA for survivin, N-cadherin, integrin β1 and interleukin-6 were increased in the ectopic endometrium of MET-treated versus vehicle-treated mice (P < 0.05). CB 1 -/- recipients that received endometrial tissue fragments from CB 1 -/- donors, WT recipients that received endometrial tissue fragments from CB 1 -/- donors and CB 1 -/- recipients that received endometrial tissue fragments from WT donors all showed a significant reduction in total lesion volume and lower expression of survivin and N-cadherin compared with WT recipients receiving uterine fragments from WT donors (P < 0.05). LARGE SCALE DATA N/A. LIMITATIONS, REASONS FOR CAUTION We provide evidence that endocannabinoid signaling via CB 1 receptor plays a role in the development of endometriosis in a mouse model. However, the relative contribution of the CB 1 -mediated signaling pathways active in inflammatory, uterine and peritoneal cells remains to be ascertained. Since the study was performed in a mouse model, the significance of the findings in the human system warrants further investigation. WIDER IMPLICATIONS OF THE FINDINGS Clarifying the function and regulation of CB 1 and its molecular interactions with endogenous ligands, and how endocannabinoids levels are regulated in women with endometriosis, represent critical areas of research for the potential development of a novel medical treatment of the disease. © The Author 2016. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology.