Background Previous results have shown tissue constriction in allergic animals following inhalation of an antigen. Further studies have demonstrated a differing response pattern in airway and parenchymal mechanics following inhaled (i.h.) or intravenous (i.v.) delivery of methacholine (MCh).Object ive The purpose of this study was to compare the acute allergic response in airway and parenchymal mechanics following i.h. and i.v. antigen challenge.Methods Brown Norway rats were sensitized to ovalbumin (OVA). Rats were anaesthetized, paralysed, and thoracotomized,and lung input impedance (ZL) between 0.5 and 21 Hz was measured using small-amplitude pseudo-random oscillations at control, after saline, and for up to 1 h after either i.h. (n = 7) or i.v. (n = 5) administration of OVA. ZL was evaluated in terms of airway resistance (Raw) and inertance (Iaw), and a constant phase tissue parenchymal damping (G) and elastance (H).Results Following i.h. OVA challenge elevations were found in Raw [192 +/- 32(SE) %], G (223 +/- 21%), and H (141 +/- 5%). Raw showed higher elevation after i.v. challenge (418 +/- 57%), whereas the elevation in G (278 +/- 30%) and H (130 +/- 4%) was approximately equal to those seen following inhalation of an antigen.Conclusions Delivery (i.v.) of an antigen produces a significantly higher response in airway resistance, whereas inhaled antigen results in a mixed airway and parenchymal response.