Monocrotaline (MCT) produces respiratory dysfunction, pulmonary hypertension (PH), and right ventricular hypertrophy (RVH) in rats. Tachykinins, such as substance P (SP) and neurokinin A (NKA), may mediate these effects. The purpose of this study was to investigate the length of tachykinin depletion (via capsaicin treatment) is needed to prevent (or attenuate) PH and/or RVH. Six groups of rats were injected subcutaneously with saline (3 ml/kg); capsaicin followed by saline or MCT (60 mg/kg); or MCT followed 7, 11, or 14 days later by capsaicin. Capsaicin (cumulative dose, 500 mg/kg) was given over a period of 4-5 days. Respiratory function, pulmonary vascular parameters, lung tachykinin levels, and tracheal neutral endopeptidase (NEP) activity were measured 21 days after MCT or saline injection. Capsaicin significantly decreased lung levels of SP but not NKA. Both capsaicin pretreatment and posttreatment blocked the following MCT-induced alterations: increases in lung SP and airway constriction; decreases in tracheal NEP activity and dynamic respiratory compliance. Administration of capsaicin before or 7 days after MCT blocked MCT-induced PH and RVH. The above data suggest that the early tachykinin-mediated airway dysfunction requires only transient elevated tachykinins, while progression of late tachykinin-mediated effects (PH and RVH) requires elevated tachykinins for more than one week.