This study was aimed at providing new knowledge on the pathology of Infantile Hypertrophic Pyloric Stenosis (IHPS) with some recently developed techniques and hence contributing to the understanding of its unsolved aetiology. Biopsy specimens of the pylorus were obtained from 15 children with IHPS at operation and 6 normal children at autopsy and compared as follows: (a) the presence or absence of muscle hyperplasia in IHPS was studied with special chromatin stain. No mitotic figures were observed in 10,000 cells screened; (b) Using the Schneider procedure, specimens from IHPS showed 1.47 +/- 0.09 mg DNA/g wet tissue (mean +/- SEM) compared with 2.38 +/- 0.18 mg DNA/g wet tissue in normal, defining the magnitude of muscular hypertrophy objectively to be in the region of 1.62 times normal; (c) Cholinesterase staining revealed adequate density of ganglia with no gross distortion of morphology in IHPS; (d) Immunocytochemical study with the marker neurone-specific-enolase confirmed that ganglia in IHPS were mature; (e) Immunocytochemical study with substance P revealed rich peptidergic innervation of the normal pylorus and a relative paucity in IHPS. This was interpreted as a phenomenon of exhaustion of substance P-neurones in IHPS. In conclusion, IHPS is a true muscle hypertrophy with little or no hyperplasia. The cholinergic ganglia are adequate and neural elements in general are sufficiently mature. The novel idea of possible disturbances of peptidergic innervation contributing to its aetiology has received some support.