The major isoform of nitric oxide synthase (NOS) in skeletal muscle is the splice-variant of neuronal (n)NOS, termed nNOSµ. Exercise training increases nNOSµ protein levels in rat skeletal muscle but data in humans are conflicting. We performed two studies to determine (1) whether resting nNOSµ protein expression is greater in skeletal muscle of 10 endurance-trained athletes compared with 11 sedentary individuals (Study 1), and (2) whether intense short-term (10 days) exercise training increases resting nNOSµ protein (within whole muscle and also within type I, IIa, and IIx fibers) in eight sedentary individuals (Study 2). In Study 1, nNOSµ protein was ~60% higher (P<0.05) in endurance-trained athletes compared with the sedentary participants. In Study 2, nNOSµ protein expression was similar in type I, IIa, and IIx fibers before training. Ten days of intense exercise training significantly (P<0.05) increased nNOSµ protein levels in type I, IIa, and IIx fibers, a finding that was validated using whole muscle samples. eNOS and iNOS protein were barely detectable in the skeletal muscle samples. In conclusion, nNOSµ protein expression is greater in endurance-trained individuals when compared with sedentary individuals. Ten days of intense exercise is also sufficient to increase nNOSµ expression in untrained individuals, due to uniform increases of nNOSµ within type I, IIa, and IIx fibers.