The time course of loss of the 55,000-Da intermediate filament protein desmin was measured in rabbit muscles subjected to cyclic eccentric contraction. Rabbit extensor digitorum longus (EDL) and tibialis anterior (TA) muscles were examined 5 or 15 min after eccentric exercise and 1 h or 1 day after 30 min of an eccentric exercise protocol (n = 16 rabbits). The earliest change noted was a significant loss of desmin labeling in 2.5 +/- 0.63% of the rabbit EDL muscle fibers (P < 0.005) 5 min after initiation of eccentric exercise. Some loss of TA fiber desmin was also apparent at this time period (0.24 +/- 0.19%), but the magnitude was not significantly different from zero (P > 0.2). Fifteen minutes after initiation of exercise, desmin loss was more pronounced, increasing to 7.4 +/- 1.4 and 4.6 +/- 1.0% in the EDL and TA, respectively (P < 0.005). Finally, 1 day after 30 min of eccentric exercise, the percentage of fibers without desmin staining rose to 23.4 +/- 3.7 and 7.7 +/- 2.4% in the EDL and TA, respectively (P < 0.001). Loss of desmin staining occurred in the absence of contractile or metabolic protein disruption. Increased staining intensity of the intrasarcomeric cytoskeletal protein titin and an inability to exclude plasma fibronectin were also observed in most but not all fibers that had lost desmin staining. Desmin disruption thus represents a very early structural manifestation of muscle injury during eccentric contraction. Cytoskeletal disruption may predispose the contractile apparatus to previously reported structural damage.