Expression of nitric oxide synthase (NOS) was studied in nine human neuroblastoma and two human glioblastoma cell lines. Neuronal NOS (n‐NOS) mRNA of ∼10 kb was detected in four of the nine neuroblastoma cell lines by northern blot analysis using human n‐NOS cDNA as a probe. Expression of the n‐NOS mRNA was also detected in another neuroblastoma cell line in a subsequent reverse transcriptase polymerase chain reaction (RT‐PCR) study, but no n‐NOS mRNA expression was observed in the other four neuroblastoma cell lines or in the glioblastoma cell lines. The level of NOS activity correlated well with that of n‐NOS mRNA expression in neuroblastoma cell lines expressing n‐NOS mRNA. Western blot analysis showed that the n‐NOS expressed in neuroblastoma cells was a 160‐kDa protein reacted with anti‐n‐NOS antibody. By using the RT‐PCR method, a short n‐NOS (n‐NOS‐2) mRNA with a 315‐bp inframe deletion from the entire n‐NOS (n‐NOS‐1) mRNA was detected in the human neuroblastoma cells. The structural diversity of human n‐NOS mRNA was demonstrated for the first time.