It has been postulated that the degenerative process in dystrophic muscle results from increased concentrations of free radicals, eroxides, or lipid hydroperoxides. Therefore, the reduction of the free radical tanol (2,2,6,6-tetramethyl-4-iperidinol-1-oxyl) by extracts of muscles of dystrophic and normal chickens was studied. Pectoral (white) and thigh (red) muscles were used. For initial rate measurements, the various muscle extracts were added to an equal volume of 0.2 mM tanol. Reaction mixtures were introduced into the EPR cavity in a standard aqueous flat cell. Rates were measured by continuously monitoring the decrease in signal amplitude of the center (MI = 0) solution tanol EPR resonance line (in-phase first harmonic absorption signal). With extracts from dystrophic white muscle, the reduction rate was 75% faster than normal, whereas in dystrophic red muscle extracts the rate was normal. This agreed with previous observations that white muscle is more severely affected than red in dystrophic chickens. The primary reductant was identified as reduced ascorbic acid, and the rate of reduction of tanol correlated directly with the concentrations of ascorbic acid in the various muscle extracts as shown by chemical analysis. The results suggest an involvement of the intracellular redox status in the pathogenesis of avian muscular dystrophy. Avian (chicken) muscular dystrophy has been considered as a model for human muscular dystrophy, in particular Duchenne dystrophy (1). Avian dystrophy is characterized: (i) by muscle weakness and atrophy, which is apparent when the bird is placed on its back and cannot right itself; (ii) by contractures or stiffness of the wings, which cannot be elevated beyond a horizontal plane; and (iii) biochemically, by increased levels of muscle enzymes in the serum, for example, creatine kinase and aldolase (1). The chicken is a good model because of the natural anatomical separation of the white, fast fibers (pectoral) and red, slow fibers (thigh and gastrocnemius). In avian and Duchenne dystrophy, the white muscle fibers are much more severely affected than the red (1, 2).It has been postulated that the degenerative process in dystrophic muscle may result from increased concentrations of deleterious agents such as free radicals, peroxides, and lipid hydroperoxides (3, 4). These agents promote lipid peroxidation,