Specific antibodies against structural proteins of muscle fibres (actin, desmin, dystrophin) and extracellular matrix (fibronectin) were used to study the effect of eccentrically biased downhill running exercise (13,5 degrees, 17 m min(-1), 130 min) on the magnitude and properties of myofibre injury in the quadriceps femoris muscle of male and female rats. Muscle beta-glucuronidase activity, a quantitative indicator of muscle damage, showed clearly smaller increase in female than in male rats during the 4-day period following exercise. A similar course of histopathological changes was observed in both sexes, although females showed slower and less marked changes than males. In males, discontinuous or even lost submembrane protein dystrophin staining was observed in some swollen fibres immediately after exercise, before the loss of desmin and staining of disorganized actin, i.e. before the disruption of the cytoskeletal system and the contractile apparatus. The observation that no dramatic changes in the microarchitecture of the muscle fibres were detected immediately or even 6 h after the exercise in females compared with males may indicate that the sarcolemma of the females might be strengthened against membrane damage by a still unknown stabilizing compound.