To investigate the effects of mechanical deformation on matrix degradation in fibrous joints, coronal suture explants from neonatal rabbits were stressed in vitro for 24 hours in an established tooth-movement model system. The metalloproteinase collagenase (CL) and its inhibitor, TIMP (tissue inhibitor of metalloproteinases), were immunolocalized in two ways by a two-step indirect technique: (1) extracellularly by immunoprecipitation at the site of secretion, and (2) intracellularly by incubation of the explants with the ionophore monensin. Immunoprecipitates of CL and TIMP were distributed throughout the sutural and periosteal tissues of nonstressed explants. In stressed explants, however, CL immunoprecipitates were predominantly associated with an area of rounded cells between the bone ends. In explants treated with monensin a significant increase in the number of CL-positive cells was observed in this cellular area; active enzyme was suggested by the demonstration of CL bound to collagen. Extracellular TIMP was not seen within the area of rounded cells of stressed explants, but intracellular TIMP was detectable; this suggests that insufficient TIMP was available to immunoprecipitate with anti-TIMP, probably because it had become irreversibly complexed with active CL. Since the area of rounded cells corresponds to the site of increased cell proliferation in this and other animal models of tooth movement, these data suggest that collagenase production and cell proliferation might be correlated. We speculate that matrix degradation is an essential prerequisite for cell proliferation as it creates room to accommodate an increase in cell population.