The first experiment examined the effects of 0.5 g/kg bwt and 1.0 g/kg bwt doses of sodium bicarbonate in 2 litres of water, administered by nasogastric tube, on venous blood bicarbonate concentration [HCO3−] in 12 Thoroughbred horses. The mean ± s.e. increase in [HCO3−] was 7.6 ± 0.4 mmol/l after the 0.5 g/kg dose and 10.1 ± 0.3 mmol/l after 1.0 g/kg. In a second experiment, the performance effects of sodium bicarbonate (1.0 g/kg bwt) and associated post exercise venous [HCO3−] were examined in horses given a high speed exercise test. Seven Thoroughbred horses received sodium bicarbonate (1.0 g/kg bwt) and venous [HCO3−] was measured after intense exercise (115% V̇O2max) on a treadmill. Blood bicarbonate concentrations were significantly higher in the treated horses for 3 h after exercise. The administration of sodium bicarbonate did not affect the run time to fatigue. We concluded that sodium bicarbonate (0.5 and 1.0 g/kg bwt) produced significant metabolic alkalosis, which persisted for up to 20 h after bicarbonate administration; and that post race sampling may allow detection of horses that have received sodium bicarbonate.