Nitric oxide (NO) has been described as a mediator of smooth muscle relaxation in the mammalian gastrointestinal tract. The enzyme neuronal nitric oxide synthase (NOS) catalyzes the formation of NO. We examined the expression of the neuronal NOS gene at the messenger RNA (mRNA) level in pyloric smooth-muscle biopsy specimens from six patients with infantile hypertrophic pyloric stenosis (IHPS) using a reverse transcription-polymerase chain reaction (RT-PCR) technique. For controls, smooth-muscle layer specimens of pylorus (n=3), ileum (n=2), and colon (n=2) were used. With 31 cycles of PCR reaction, control specimens revealed detectable signals for neuronal NOS mRNA. In contrast, signals of IHPS specimens were undetectable in five cases and very weak in one. By increasing the PCR to 37 cycles, detectable signals for neuronal NOS mRNA were observed in all IHPS specimens, but they were significantly weaker than those of controls. Since a low level of neuronal NOS mRNA may lead to impaired production of NO, our observations indicate that the excessively contracted, hypertrophied pyloric muscle in IHPS is a result of reduced expression of the neuronal NOS gene at the mRNA level.