In vivo and in vitro aging of tendon from rat tail, kangaroo tail and human wrist tendon was examined by the technique of isometric melting, in physiological saline. 2. For all these collagens, two mechanisms of structure stabilisation can be distinguished in the melting curves. One of these involves co-valent cross-linking as judged by its increasing stability to heat and acid pH, while the second appears to involve only secondary interactions. 3. The time rate of the first process is slow in vivo; rat tendon up to 2 years does not show it, but it is present in 6-year-old human tendon. However, its in vitro rate is markedly dependent upon the free oxygen content of the physiological saline. At an oxygen concentration of 300 nmol/ml, the in vitro aging rate is about 30 times the in vivo rate for rat tail tendon, and about 20 times for both kangaroo tail tendon and human wrist tendon. At a concentration of 60 nmol/ml (which is about the same as normal arteriovenous blood difference) in vitro aging proceeds close to the in vivo rate.
