Poloxamer 188 NF (national formulary (NF) grade of P-188) improves cardiac muscle function in the mdx mouse and golden retriever muscular dystrophy models. However in vivo effects on skeletal muscle have not been reported. We postulated that P-188 NF might protect diaphragm muscle membranes from contraction-induced injury in mdx and mdx/utrophin-/- (dko) muscular dystrophy models. In the first study 7-month old mdx mice were treated for 22 weeks with subcutaneous (s.c.) injections of saline or P-188 NF at 3 mg/Kg. In the second, dkos were treated with saline or P-188 NF (1 mg/Kg) for 8 weeks beginning at age 3 weeks. Prednisone was the positive control in both studies. Respiratory function was monitored using unrestrained whole body plethysmography. P-188 NF treatment affected several respiratory parameters including tidal volume/BW and minute volume/BW in mdx mice. In the more severe dko model, P-188 NF (1 mg/Kg) significantly slowed the decline in multiple respiratory parameters compared with saline-treated dko mice. Prednisone's effects were similar to those seen with P-188 NF. Diaphragms from P-188 NF or prednisone treated mdx and dko mice showed signs of muscle fiber protection including less centralized nuclei, less variation in fiber size, greater fiber density, and exhibited a decreased amount of collagen deposition. P-188 NF at 3 mg/Kg s.c. also improved parameters of systolic and diastolic function in mdx mouse hearts. These results suggest that P-188 NF may be useful in treating respiratory and cardiac dysfunction, the leading causes of death in Duchenne muscular dystrophy patients.