Respiratory inductance plethysmographic (RIP) and pneumotachographic (Pn) flows were compared dynamically in horses with bronchoconstriction. On a breath-by-breath basis, RIP was normalized to inspiratory volume from Pn, and peak [peak of subtracted final exhalation waveform (SFE(max))] and selected area [integral of subtracted final waveform during first 25% of exhaled volume (SFE(int))] differences between RIP and Pn flows during early expiration were measured in three settings: 1) healthy horses (n = 8) undergoing histamine bronchoprovocation; 2) horses with naturally occurring lower airway obstruction (AO) (n = 7); and 3) healthy horses (n = 6) given lobeline. HCl to induce hyperpnea. In setting 1, histamine challenge induced a dose-dependent increase in SFE(max) and SFE(int) differences. A test index of airway reactivity (interpolated histamine dose that increased SFE(max) by 35%) closely correlated (r(s) = 0.93, P = 0.001) with a conventional index (histamine dose that induced a 35% decrease in dynamic compliance). In setting 2, in horses with AO, SFE(max) and SFE(int) were markedly elevated, and their absolute values correlated significantly (P < 0.005) with pulmonary resistance and the maximum change in transpulmonary pressure. The effects of bronchodilator treatment on the SFE(max) and SFE(int) were also highly significant (P < 0.0001). In setting 3, hyperpnea, but not tachypnea, caused significant (P < 0.01) increases in SFE(max) but not in SFE(int). In conclusion, dynamic comparisons between RIP and Pn provide a defensible method for quantifying AO during tidal breathing, without the need for invasive instrumentation.