Eight horses were exercised at speeds to produce 80% of maximal heart rates, on 4 occasions using a randomised block design, and girthed at tensions of approximately 5, 10, 15 or 20 kg using a standard canvas girth attached to a racing saddle. Tension was recorded continuously using an in-line load cell connected to a physiograph. Horses ran until fatigued on a treadmill inclined at 10% slope. Tensions were measured at peak inhalation (T/inh) and exhalation (T/exh), recorded at rest (rest) and during exercise (ex). Data were analysed by ANOVA and, in addition, least squares linear regression was performed to determine the relationship between distance travelled, time to fatigue and the recorded maximum tensions at inhalation and minimum tensions at exhalation. For T/inhrest, every increase in girth tension by 1 kg was associated with a reduction in distance to fatigue by 98.3 +/- 28.6 m and a corresponding reduction in run times of 0.24 +/- 0.07 min (P = 0.01). Results for T/exhrest were similar, with a reduction of mean +/- s.e. 81.3 +/- 26.9 m and 0.20 +/- 0.07 min per 1 kg increase in tension (P = 0.02). The mean distances to fatigue for the T/exhrest of 5, 10, 15 and 20 kg were 6624, 5812, 5268 and 5405 m respectively. These data indicate that distances to fatigue are similar once T/exhrest equals or exceeds 10 kg. Based on this study, resting girth tensions less than 10 kg are optimal for performance, but from our survey data, tensions exceeding this tension are typical for Thoroughbred racing in Victoria.