The neuronal form of nitric oxide synthase (nNOS) is responsible for the production of NO, which acts as a neurotransmitter for penile erection and urethra relaxation. An nNOS splice variant form, nNOS-μ, was first reported to be specifically expressed in skeletal muscle and heart in the rat, but later also identified in rat penile cavernosum. We report here an apparently universal expression of nNOS-μ mRNA in rat tissues, including brain, which was previously reported to be lacking nNOS-μ. Immunoblot analysis revealed that some commercially available nNOS antibodies had high levels of nonspecific activities, which could lead to the appearance of seemingly multiple forms of nNOS. Immunohistochemical analysis with these antibodies also produced nonspecific stainings. In humans, nNOS-μ expression appeared to be confined to skeletal muscle and heart. Human penile tissues obtained from patients with erectile dysfunction did not express nNOS-μ. The human nNOS-μ-specific cDNA sequence was 89% homologous to its rat counterpart.