Beginning on day 4 ex ovo, and every 3 d thereafter, genetically dystrophic Line 413 chickens were given intraperitoneal injections (4 mg/kg body wt) of a protease inhibitor, leupeptin, pepstatin, or antipain. Experimental chickens received protease inhibitors dissolved in a water:ethanol:dimethyl sulfoxide solution (50:40:10, vol:vol:vol). Control untreated animals received diluent injections. Untreated dystrophic chickens typically reach around day 30 ex ovo a maximum ability to right from the supine position in a standardized functional test for muscle weakness. After day 30 ex ovo, the dystrophic chickens are found to decline progressively in their ability to right, compared with normal, nondystrophic controls, which have an unimpaired ability to right. Concomitantly, dystrophic chickens exhibit characteristically high levels of plasma creatine phosphokinase enzyme activity. In addition, an increased frequency of degenerating, regenerating, and vacuolated myofibers, and inflammatory cells appear in the affected pectoralis major muscles from the dystrophic chicken. Throughout the duration of the trial, there was no major enhancement in the functional righting ability of dystrophic chickens receiving any one of the protease inhibitors tested. However, there was a significant reduction in the abnormally high levels of plasma creatine phosphokinase in the treated chickens. Also, there was an apparent reduction in the mean number of vacuolated fibers in the pectoralis muscle from the protease inhibitor-treated birds. No significant reductions were observed in the relative frequency of degenerating and regenerating myofibers or inflammatory cells. In addition to the plasma creatine phosphokinase decrease, however, therapeutic benefit was seen in 31.0, 30.5, and 14.8% increases in the wet weight (and total noncollagen protein) of pectoralis muscle from dystrophic chickens receiving leupeptin, pepstatin or antipain, respectively.