Apoptotic processes are often associated with an intense proteolytic remodeling of the extracellular matrix (ECM). Proteolytic degradation of the ECM can also be a signal that induces apoptosis. Here, we have investigated the expression pattern and functional role of the matrix metalloproteinase stromelysin-3 in follicular atresia. Twenty-four hours after the treatment of immature female mice with a low dose of eCG, both apoptosis and the stromelysin-3 mRNA expression were suppressed approximately threefold. However, the initial suppression of apoptosis and stromelysin-3 expression was followed by a time-dependent increase, and 96 h after eCG treatment, the levels were similar to those of untreated control mice. In 15- to 16-day-old juvenile mice, the ovary consisted of relatively undeveloped follicles, and almost no apoptosis and only low stromelysin-3 mRNA expression were observed. However, at the age of 21 days, when several antral follicles were present, a fivefold induction in both apoptosis and stromelysin-3 mRNA expression was detected. For both models, in situ analysis revealed that the expression of stromelysin-3 mRNA was localized to the granulosa cells of atretic follicles. To address the functional role of stromelysin-3 in follicular atresia, stromelysin-3-deficient mice were studied. However, no difference in the pattern of apoptotic DNA fragmentation and no apparent morphological differences were observed when ovaries from wild-type and stromelysin-3-deficient mice were compared. Taken together, our data indicate that stromelysin-3 is induced during follicular atresia, but that this protease is not obligatory for initiation or completion of the atretic process.