Aims: To study the effect of diet hardness on condylar cartilage thickness, extracellular matrix composition, and expression of matrix metalloproteinase (MMP) -3, -8 and tissue inhibitor of metalloproteinase-1 (TIMP-1), by using immunohistochemical and morphometric methods. Methods: Seventy-two female Sprague Dawley rats were exposed to different dietary hardness, from soft to hard. MMP -3, -8, and TIMP-1 expression, cartilage thickness, cell count, and expression of type II collagen were studied. Analysis of variance among treatments was carried out followed by Bonferroni's comparisons test. Results: The ratio of MMP-3 and TIMP-1 immunopositive cartilage cells were similar in all age groups, whereas the number of MMP-8 positive cells decreased with age. A change of diet from soft to hard caused a significant decrease in the number of MMP-3 and MMP-8 and an increase in TIMP-1 positive cells. Cartilage thickness and area of type II collagen-positive staining were significantly affected by diet hardness. Conclusion: The results show that a soft diet during growth increases collagenolytic activity and may increase the vulnerability of condylar cartilage. Keywords: condylar cartilage, diet hardness, mandibular condyle, MMP, TIMP