Native and reconstituted rat tail tendon collagen were tested mechanically after in vitro maturation by incubation. The mechanical strength of the native tendons increased upon incubation and attained maximum strength values similar to those of tendons matured and aged in vivo. This finding indicates that the same stabilizing process occurs both in vivo and in vitro. However, the mechanical strength values similar to those of tendons matured and aged in vivo. This finding indicates that the same stabilizing process occurs both in vivo and in vitro. However, the mechanical strength increased at an initial higher rate in vitro than in vivo. The mechanical strength of fibrils reconstituted from purified tail tendon collagen increased during incubation in air as previously reported for fibrils prepared from skin collagen. Fibrils prepared from tail tendon and skin collagen shared common mechanical and thermal stability characteristics upon the incubation. However, distinct qualitative mechanical characteristics for fibrils of the two collagens were found. These characteristics showed a resemblance to those of the respective source tissues. The results indicate that the same process is responsible for the gain in mechanical strength of native tissues and reconstituted collagen fibrils. Thus, reconstituted collagen fibrils seem a useful model for studying mechanical stability changes during maturation of collagen.