Biochemical and morphological aspects of fibrocartilages of calcaneal and deep digital flexor tendons in rats aged 30, 180 and 730 days were analyzed. In both tendons a stronger staining with Alcian blue, indicating the presence of proteoglycans, was detected in rats of 30 and 180 days. In animals 730 days old, it was restricted to the pericellular area. Ultrastructural analysis showed a more prominent pericellular matrix in calcaneal tendon compared to the deep digital flexor tendon. The biochemical analysis showed higher levels of proteins and glycosaminoglycans in the calcaneal tendon of 30-day-old rats compared to older rats. In the deep digital flexor tendon, no significant differences were observed between ages. The small proteoglycan, fibromodulin, was detected in both tendons of all ages, but in young rats it appeared to be running as a 210 kDa component, probably due to the association with collagen chains or self-association. (c) 2006 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved.316570577