We studied in vivo the effects of pre-inhalations of vasoactive intestinal polypeptide (VIP) and a precursor analogue of VIP (Leu17 VIP-Gly-Lys: preVIP) in mongrel dogs bronchoconstricted by ascaris. An inhalation of preVIP solution (2 mg/5 ml saline) gave significant protection for 120 min against increases in respiratory resistance (Rrs) and decreases in dynamic compliance (Cdyn) induced by ascaris challenges. An inhalation of VIP solution (2 mg/ml saline) also gave significant protection for 120 min against increases in Rrs induced by ascaris challenges. Protective effect of the inhalation of preVIP against ascaris-induced bronchoconstriction was more potent than that of VIP. The inhalations of preVIP and VIP solution did not change systemic blood pressure or heart rate. These results indicate that inhalations of preVIP or VIP solution would attenuate ascaris-induced bronchoconstriction in dogs without affecting cardiovascular dynamics.