AIM: To explore the regulatory mechanism of nerve growth factor (NGF) on substance P in the experimental asthmatic guinea pig. METHODS: Radioimmunoassay was used to determine the alteration of substance P levels in the lower respiratory tract and visceral sensory afferent sites while NGF was absent (inhalation of NGF antibody through nasal cavity) in the asthmatic guinea pig. RESULTS: The contents of substance P in the trachea, bronchus, lung, C7 - T5 spinal ganglia and the correspondent spinal dorsal horn, nodose ganglia and solitary nucleus area in the experimental asthmatic guinea pig with the absent of NGF in the respiratory tract, were much lower than those in the asthmatic and control groups (P