Basal airway smooth muscle (ASM) tone has not been demonstrated in mice in vivo. To determine whether basal ASM tone is present in mouse airways we measured respiratory system impedance (Zrs) before and after either atropine or bilateral vagotomy. Zrs was measured using forced oscillations delivered via a wave-tube during slow (similar to 35 s) inflation-deflation maneuvers between transrespiratory pressures (Prs) of 0 and 20 cm H2O. A constant-phase tissue model was applied to the Zrs to calculate airway resistance (R-aw), tissue damping (G) and elastance (H). Thoracic gas volume (TGV) was determined plethysmographically at Prs = 0 cm H2O and by integration of the inspiratory flow. The relationship between conductance (G(aw) = l/R-aw) and TGV during inflation was also examined. Neither atropine nor vagotomy produced any change in R-aw, H, eta (=G/H), TGV or the slope of G(aw) vs. TGV that was different to that observed in the relevant control groups. These data show that BALB/c mice do not have cholinergic ASM tone in vivo. (C) 2008 Elsevier B.V. All rights reserved.