AIMS: To examine the expression of aggrecanase-1 and a tissue inhibitor of metalloproteinases (TIMP-3) in the condylar cartilage of young rats and to determine their relationship during altered dietary loading at different time points after weaning. METHODS: One hundred Sprague-Dawley rats were randomly assigned to 1 of 2 groups: the soft-diet group, which served as the control group (n=50), or the hard-diet group, which served as the experimental group (n=50). Ten soft- and 10 hard-diet rats were killed at 6 hours, 12 hours, 24 hours, 48 hours, and 9 days after weaning (i.e., after initiation of diet change for hard-diet rats). The right-side temporomandibular joints (TMJs) were prepared for immunohistochemical staining. The cartilage from the left-side mandibular condyles of all 10 animals in each group was combined for Western blot analysis. RESULTS: Immunohistochemical analysis revealed strong staining for aggrecanase-1 localized mainly in the chondrocytes of proliferative and upper hypertrophic cartilage zones at all time points in both groups. The immunohistological expression of aggrecanase-1 was significantly higher in the hard-diet group at 12 and 24 hours than in the soft-diet group. Strong staining for TIMP-3 was mainly localized in the chondrocytes of proliferative and upper hypertrophic zones at all time points in both groups. The expression of TIMP-3 in the hard-diet group was at a significantly lower level compared to the soft-diet group at 6 hours. Western blot analysis also showed time-related differences in aggrecanase-1 and TIMP-3, but there was no significant difference between the 2 groups. CONCLUSION: The temporary change in aggrecanase-1 and TIMP-3 expression reflects the complex interaction of these enzymes in the physiologic range and cartilage response to altered dietary loading.