During the menstrual cycle, dynamic morphological changes are observed in the ovarian follicle and the endometrium. These changes are associated with the onset of the inflammatory response in which many proteinases play various roles. Thrombin-induced activation of PAR₁ (proteinase-activated receptor 1) stimulates the production of interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) in human granulosa cells, suggesting a possible role for PAR₁ in the ovulatory process. In the endometrium, PAR₂ expression increases during the menstrual period. PAR₂ activation induces IL-8 production and cell proliferation in human endometrial stromal cells. PAR₁ also stimulates proinflammatory cytokine production in human endometrial stromal cells. Thus, the PARs may be important in directing the dynamic changes of the endometrium. PARs also appear to play a role in endometriosis, a common gynecological disease, since activation of PAR₁ and PAR₂ induces the secretion of inflammatory cytokines and the proliferation of stromal cells in endometriotic lesions. Taken together, PARs appear to play diverse roles in the human reproductive organs.