Matrix metalloproteinase 1, MMP-1, which was previously called interstitial collagenase, is necessary for extracellular matrix reconstruction. The immunolocalization of the latent form of MMP-1 (proMMP-1) was examined in the ovulatory process in hCG-treated (100 iu per animal) rabbit ovaries. Immunoreactive products of proMMP-1 were identified by the avidin-biotin peroxidase complex formation using an anti-rabbit proMMP-1 polyclonal antibody. ProMMP-1 was distributed in the cytoplasm of theca interna cells, theca externa cells, interstitial glands and germinal epithelium throughout ovulation. However, at 9 or 10 h after hCG treatment, this enzyme was identified in several capillary lumina around the apex of preovulatory follicles. In addition, the staining density of immunoreactive products apparently increased in granulosa cells and theca interna cells around the orifice of the ruptured follicle 10 h after the stimulation. These results indicate that the spatiotemporal appearance of proMMP-1 in ovulation may be closely associated with the initiation of rupture of the follicle.