Changes in expression of type III alpha1-collagen and myosin II heavy chains were characterized in rabbit skeletal muscle following single stretch injury using quantitative reverse transcription-polymerase chain reaction. Collagen III expression was highly elevated in the injured leg compared with the control limb both at the myotendinous junction and in the distal muscle belly. While upregulation of collagen III expression at the myotendinous junction was maximal on day 1, collagen III expression in the distal muscle belly was unchanged on day 1 but highly elevated by day 3. Over the initial 7-day period, there was on average a 94% increase in collagen III expression at the myotendinous junction and a 42% increase in the distal muscle belly. On the other hand, there was little difference, in fact, slightly less expression of myosin II isoforms, in the injured leg compared with the control side. Immunohistochemical analysis of injured muscle showed significant collagen III deposition at the myotendinous junction beginning at day 3 post-injury and still evident by day 14. Focal deposits of type I and III collagen were first apparent in the distal muscle belly by day 3 and striking by day 7. Taken together, the data suggest the formation of connective tissue scar at the injury site and the absence of significant muscle regeneration following muscle stretch. Furthermore, microinjuries distant to the primary site of injury may result in more general muscle fibrosis and scarring.