The organisation of tendon collagen fibrils has been studied by electron microscopy after treatment with an anabolic steroid hormone in a short-term and a long-term study. The effect of anabolic steroids is of interest because of their use in competitive sports and in clinical therapy. In a functional experimental model the unexpected occurrence of dysplastic as well as ruptured and dissociated collagen fibrils was revealed. The striking time-dependent collagen dysplasia and other pathological findings strongly suggest that in mice anabolic steroid treatment predisposes to tendon rupture especially when the animals are exercised. The possible underlying mode of action of anabolic steroid hormone on connective tissue is discussed.