This study was designed to test the hypothesis that a specific beta 2 adrenergic receptor agonist, albuterol sulphate, when delivered by metered-dose inhaler (MDI) would affect the performance of Thoroughbred horses during incremental exercise testing. Six conditioned Thoroughbred horses were randomly assigned to one of 2 treatment groups: Group 1 horses received placebo and Group 2 received albuterol. Each horse received both treatments in a crossover design with a 3 week interval between trials. Horses inspired albuterol (900 micrograms) or placebo over a 5 min period followed by an incremental, peak oxygen consumption (VO2peak) exercise test. Facial arterial blood gases (PaO2, PaCO2), pHa, temperature, haemoximetry (O2Hb, COHb, MetHb, RHb) and electrolytes (Na+, K+, iCa2+, Cl-) were measured. Heart rate (HR), VO2peak, CO2 production (VCO2), plasma lactate, speed at failure, exercise duration, and urine levels of albuterol and metabolites (Alburine) were monitored. The Paratrend7 intravascular sensor provided continuous arterial blood gas data. Mixed effects linear modelling using generalised least-squares estimation was used to assess treatment and speed main effects interaction. Significant differences (P < 0.01) were found between placebo and albuterol-treated horses for total run time (mean +/- s.e. 405.8 +/- 8.8 vs. 430.5 +/- 6.7 s) and run time from completion of 8 m/s until fatigue (169.3 +/- 8.6 vs. 187.3 +/- 6.7 s), as well as VO2peak (121.7 +/- 3.7 vs. 130.3 +/- 3.8 ml/kg/min), respectively. In conclusion, inhaled albuterol sulphate has a small but significant effect on the performance of fit Thoroughbred horses during incremental, VO2peak exercise testing.