The purpose of this study was to examine the effect of anabolic steroid hormones and exercise training on skeletal tendons. Female mice were exercised for 1 and 10 weeks in an endurance running programme on a treadmill. The altered ultrastructure of tendons caused by simultaneously administered anabolic steroid hormone was investigated by electron microscopy. A stereoscopic analysis of collagen fibrils was performed in order to reveal the changes in the architecture of tendons and to quantify the extent of possible injuries to their functional structure. The occurrence of tendon injuries was detected by the appearance of a collagen dysplasia. The morphometric analysis of the degree of the collagen dysplasia in the different experimental groups reveals that it becomes even more conspicuous in the tendons of the mice which were both exercised and given anabolic steroids. The overall picture and the architecture of the tendons provide tentative evidence that anabolic steroid hormones may induce tendon injuries, depending on the duration of treatment. The direct effect of anabolic steroids on tendons has to be taken into account when considering the clinical disorders of tendons and ligaments which occur in high-class athletes.