The distal diaphyseal shafts of the rat tibia and fibula fuse postnatally, a process initiated by the seventh day with the formation of secondary cartilage that is subsequently replaced by endochondral ossification. The histological appearance of this fusion process is described. A homologous process occurs postnatally in the mouse. An analysis of some of the pertinent recent developmental, comparative and paleontological data indicated that extrinsic, bio-mechanical factors probably played a significant causal role in tibia-fibula fusions. The production of secondary cartilage is evoked by extrinsic forces, and these fusions were analyzed within the present concepts of intrinsic and extrinsic factors in skeletogenesis. It was concluded that the fusions currently reported fit well within the hypothesis of the functional matrix and that, such fusions between skeletal tissues are secondary, compensatory and mechanically obligatory responses to the prior demands of functionally related nonskeletal tissues and organs.