The muscles of mdx mice lack the subsarcolemmal protein dystrophin, and as a consequence may be more susceptible to damage induced by contractions. The purpose of this study was to characterize the response of muscles in mdx mice to contraction-induced injury in situ. The hypothesis tested was that following a protocol of repeated stretches of maximally activated muscles, the magnitude of the injury is greater for muscles in mdx mice than for muscles in C57BL/10 control mice, and consequently, the muscles in mdx mice recover more slowly. Each stretch was of 20% strain relative to muscle fibre length (Lf) at 0.5 Lf s-1 and was initiated from the force plateau of an isometric contraction. The protocol consisted of a total of ten contractions, with one contraction occurring every ten seconds. The time-course of injury and recovery was determined through measurements of in situ force production at 10, 30, 45 and 60 minutes, and either 12, 24, 48 or 72 hours after the contraction protocol. The initial injury, as assessed by the decrease in force production both immediately and 60 minutes after the contraction protocol, was significantly greater for the muscles in mdx mice compared with those in control mice. Over the next three days, a value for maximum isometric force of sim 80% of the pre-injury value was maintained for muscles in control mice, whereas within three days muscles in mdx mice showed complete recovery of force. For muscles in mdx mice, the greater decrease in force during the contraction protocol and the more rapid recovery indicates an increased susceptibility to contraction-induced injury but an enhanced rate of recovery.