The fetal development of the pyloric muscle was studied in five human embryos (crown-rump length 5 to 31 mm) and in ten fetuses aged 3 to 9 months. Samples of pyloric muscle were obtained during operation for pyloric stenosis in two infants aged six weeks. Anatomo-radiologic, morphologic and immunohistochemical studies were made on this material, from which it emerged that the pylorus is identifiable by means of specific markers from the 40th day. Its two-layered muscular structure is described in detail. The mechanism of sphincteric function is reviewed. This study assumes clinical importance in the context of the etiopathogenesis of hypertrophic stenosis of the pylorus.