Objective To further study the pathogenesis of infantile hypertrophic pyloric stenosis(IHPS) by analyzing the expression of neuronal nitric oxide synthase(nNOS) in pylorus of CHPS. Methods According to 3:1,four Beagle pregnant dogs were selected and divided into model group and control group,from the first day of pregnancy to the first day before delivery, L-NAME was intraperitoneally injected into pregnant dogs in the model group, dose for 6mg/(kg.d),while normal saline was intraperitoneally injected into pregnant dogs in the control group.The two groups newborn dogs were weighed at 31, 38, 45 and 52 days after birth,At 52 days after birth, 5 newborn dogs were selected respectively between model group and control group as experimental subjects, obtained pyloric tissue for HE staining and measured the muscular thickness of pyloric,The expression of nNOS was determined by image analysis after immunohistochemistry. Results Compared with newborn dogs in the control group, newborn dogs in the model group had slow weight growth, increased pyloric annulus muscle thickness, and decreased expression of nNOS(P<0.01,P<0.01,P<0.05). Conclusion The decreased expression of nNOS in the pyloric region associated with IHPS.