The alterations caused by Achilles tenotomy in the calf muscles of the rat were studied one, two and three weeks postoperatively by scanning electron microscopy (SEM). One week after tenotomy connective tissue had accumulated in endomyseal and perimyseal structures, which continued to increase even more after two and three weeks. The normal muscle architecture was markedly disturbed and in individual muscle cells hypercontracted segments, longitudinal splitting as well as destruction of myofilaments were found. Apparently due to the loss of muscle tension some muscle cells were reversed on their long axis and fixed with the adjacent collagen fibres. Some muscle cells were spiral in shape or crossing over each other. Formation of myotubes indicated onset of regenerative processes. According to the SEM-analyses tenotomy has many severe deleterious effects on muscle structure. In clinical practice tendon ruptures should therefore be repaired as soon as possible to avoid degenerative--potentially irreversible--changes in the muscular tissue.