The effect of measurement frequency on respiratory mechanics was investigated in six horses with reversible allergic airway disease. Total respiratory impedance was measured at 1.5, 2.0, 3.0, and 5.0 Hz by using the forced oscillation technique with the horses in remission, after acute antigenic challenge producing clinical heaves, and with heaves but after the administration of 2 mg fenoterol by inhalation. The slopes of the magnitude (magnitude of Zrs) and real part (R) of total respiratory impedance over the frequency range 1.5-3 Hz changed significantly after antigenic challenge and fenoterol. The ratio of R at 2 Hz to R at 3 Hz, however, discriminated better among the three conditions. Compliance and resonant frequency (calculated by using a three-element model) changed significantly after antigenic challenge and fenoterol, but inertance did not. We concluded that horses with heaves showed frequency dependence of R and (magnitude of Zrs) at frequencies up to 3 Hz and that parameters derived from a three-element model were useful indicators of small airway obstruction in the horse.