To substantiate the role of nitric oxide synthase type-I (NOS-I) in neurogenic muscular disorders we investigated human biopsy samples of type-II fiber atrophy and amyotrophic lateral sclerosis (ALS) by NOS-I immunoreactivity (-IR), NOS-associated NADPH-dependent diaphorase activity (NOSaD) and Western blot analysis. In type-II atrophy, loss of NOSaD and reduced NOS-I-IR was apparent in atrophic myofibers. In atrophic fiber groups lacking NOSaD, both NOS-I and dystrophin-IR was decreased while sarcolemmal beta-dystroglycan- and adhalin-IR (markers of the sarcolemmal dystrophin-glycoprotein complex) was normal. In ALS, groups of scattered angulated atrophic fibers revealed partial loss of NOS-I-IR/NOSaD. Atrophied fibers of either type-I or type-II thus revealed differential sarcolemmal NOS/NOSaD pattern thereby reflecting myopathological alterations of the NO-system in human type-II atrophy and ALS.