The matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) have been postulated to play a critical role in the extracellular matrix (ECM) remodeling associated with follicular development. The gelatinases were localized to the theca of developing follicles and in the stroma of the rodent ovary. Gelatinolytic activity corresponded with the localization of MMP-2 and MMP-9 around the developing follicles and at the apex of preovulatory follicles. The TIMPs-1, -2, and -3 were localized to the stroma and theca of developing follicles and correlation between MMPs and the quality of the developing follicles was found. During the process of ovulation, MMP-1 protein was found in the theca interna and externa, interstitial glands, and germinal epithelium. Synthetic inhibitor of mammalian tissue collagenases was documented to be inhibitory to ovulation in perfused rat ovaries. MMP-19 and TIMP-1 messenger RNA were localized to the granulosa and thecal-interstitial cells of large preovulatory and ovulating follicles. Both were induced and upregulated 5-10 fold by human chorionic gonadotropin (hCG). MMP-2 mRNA found in theca-interstitial cells and membrane-type (MT) 1-MMP mRNA found in granulosa and theca-interstitial cells were both induced after stimulation with pregnant mare's serum gonadotropin (PMSG). Gelatinolytic activity was observed throughout the formation of the corpus luteum. At 12 h after hCG, luteinizing granulosa cells expressed TIMP-1 and TIMP-3 mRNA. In the newly forming corpus luteum at 24 h after hCG administration, the luteal cells expressed TIMP-1, -2, and -3 mRNA with unique pattern of cellular expression for each of the TIMPs. Regression of the corpus luteum is associated with a significant increase in the activity of the metalloproteinases. In luteinized granulosa cells from women with polycystic ovarian syndrome (PCOS) the MMP-TIMP balance is shifted towards greater MMP activity. Cultured luteinized granulosa cells obtained from PCOS patients secrete higher levels of MMP-9 and MMP-2 compared to granulosa cells from normal ovulatory patients whereas the secreted basal level of TIMP-1 was similar in both types of granulosa cells. These results indicate a higher net gelatinolytic activity within the luteinizing granulosa cells of patients with PCOS. It has been shown that in sheep, diversion of normal follicles to atresia by hypophysectomy is followed by a significant increase of intrafollicular levels of MMP-2 and MMP-9 and the disappearance of connexin-43. It is therefore reasonable to speculate that MMP-9 and MMP-2 may be associated with inappropriate atresia in PCOS.