Menstruation, angiogenesis, embryo implantation and placentation are natural processes involving degradation of the extracellular matrix within the endometrium. A number of matrix metalloproteinases (MMPs) and their natural tissue inhibitors (TIMPs) have now been identified in association with these processes. In particular, mRNA for proMMP-1 and proMMP-3 and for TIMP-1 and TIMP-2 are elevated in human endometrial tissue at menstruation compared with other times of the female reproductive cycle. ProMMPs-1, -2, -3 and -9 are released from cultured human endometrial stromal cells, production of all but proMMP-2 being regulated by cytokines known to be expressed in human endometrial epithelium. At least two gelatinases are released by ovine trophoblast in culture and can be flushed from the uterine lumen at the time of implantation. Thus, matrix metalloproteinases clearly have roles in normal endometrial functions.