[en] The performance of a Fleisch No. 5 pneumotachometer (F), and two commercial ultrasonic pneumotachometers, the BRDL (B) and the Spiroson (S) systems were compared in respect to their use for determination of ventilatory parameters at rest and during lobeline-induced hyperpnoea. Five clinically healthy Thoroughbred horses were tested with the three pneumotachometers in random order. Respiratory airflow, respired gas concentrations, oesophageal pressures, pressures within the mask systems and arterial blood gases were determined before and during lobeline-induced hyperpnoea. Because measured peak expiratory airflow rates exceeded the stated linear range of the Fleisch pneumotachometer ( approximately +/- 25 l s(-1)) differential pressure-flow curves were determined in vitro over the range of flows recorded in vivo. Expired flows greater than the linear range were corrected according to the derived regression equation. No differences in any of the measured variables among the three systems were present at rest. At peak ventilation of lobeline-induced hyperpnoea mask pressures [Delta P(mask)(mean (SEM)): F: 9.6 (2.8) cm H(2)O, B: 0.8 (0.4) cm H(2)O, S: 1.4 (0.8) cm H(2)O] and end tidal carbon dioxide [ ET CO(2)(mean (SEM)): F: 2. 6 (0.1)%, B: 2.1 (0.2)%, S: 2.1 (0.1)%] were significantly higher in system F. Despite a tendency for respiratory frequency and peak inspired and expired flows, to be lower with system F, no significant differences in the measurements of ventilatory mechanics were detected. In conclusion, the ultrasonic flowmeters pose significantly lower resistive loads onto the respiratory system during ventilation above resting levels than Fleisch No 5 pneumotachometers. However, at the flowrates achieved during lobeline-induced hyperpnoea an in vitro calibration of the differential pressure-flow relationship allows correction for expiratory alinearity in system F. In addition, the performance of the Spiroson flowmeter is accurate in determining ventilatory mechanics at rest and during lobeline-induced hyperpnoea.