We have examined the expression of the murine counterpart of human collagenase-3, a matrix metalloproteinase produced by breast carcinomas, in the course of processes which involve extensive tissue remodeling. By using Northern blot analysis, we have found that collagenase-3 is expressed in the rat ovary, but not in the remaining analyzed tissues including brain, kidney, liver, lung, mammary gland, uterus, bladder, heart, intestine, prostate, spleen, testis and thymus. Collagenase-3 mRNA was detected at high levels in rat ovaries at proestrus and estrus, was at a minimum at metestrus and started to increase during diestrus through to proestrus. In addition, collagenase-3 was also detected on day 21 of pregnancy, which is approximately one day before parturition. However, no significative expression was detected in RNA from ovaries taken immediately after parturition, or on days 1, 5 or 30 postpartum. Northern blot analysis also revealed that collagenase-3 was not expressed at significant levels, compared with ovarian expression, in the uterus or in the mammary gland during pregnancy or after parturition. When follicular granulosa cells were separated from residual ovarian tissue and their RNA was analyzed by Northern blot, it was seen that collagenase-3 was not expressed by the granulosa cells but was present in the residual tissue containing interstitial and thecal tissues, growing follicles and corpora lutea. Immunohistochemical studies also confirmed, at the protein level, the localization of collagenase-3 in rat ovary. Gonadotropic stimulation of ovulation in immature rats by priming with pregnant mare's serum gonadotropin and stimulation with human chorionic gonadotropin failed to induce the expression of collagenase-3, suggesting that additional factors which are not present in the immature stimulated rats are needed for completely effective induction of the expression of this matrix metalloproteinase. On the basis of these results, together with the comparative analysis of expression of different matrix metalloproteinases in the rat ovary, we propose that collagenase-3 is a major ovarian metalloproteinase potentially involved in ovarian function during the reproductive cycle. Journal of Endocrinology (1996) 149, 405–415