[NO] was measured in sera that were collected from sedentary mice, or mice immediately after completion of treadmill running (acute), or voluntary running for 14 days (chronic). The lack of difference in serum [NO] in sedentary wild-type (n= 14), Tg–/mdx (n= 6), nNOS null (n= 6) or Tg+/mdx (n= 6) mice shows that nNOS-derived NO does not contribute significantly to serum NO in sedentary mice. Ten mice were analysed in the Tg+ group. However, acute activity increased serum [NO] in wild-type (n= 20), Tg+/mdx (n= 6) and Tg+ (n= 10) mice, but not in Tg–/mdx (n= 6) mice, showing that muscle nNOS elevates serum [NO] in active mice. Voluntary running for 14 days increased serum [NO] in Tg+/mdx (n= 6) mice, but not in wild-type (n= 6) or mdx mice (n= 6). #Differs significantly from mdx under same treatment conditions. *Differs significantly from sedentary mice of same genotype.