Six detrained Thoroughbred horses performed incremental treadmill tests. Maximal oxygen uptake (VO2max) was measured during exercise by analysis of expired gas. The relationships between oxygen consumption (VO2) and work rate, heart rate (HR), cardiac output (Q), stroke volume (SV) and arteriovenous oxygen content difference [C(a-v)O2] were examined during submaximal and maximal exercise. The relative contributions of blood flow and extraction of oxygen from muscle capillaries were assessed during exercise at VO2max. Mean VO2max was 129.7 +/- 2.9 (mean +/- S.E.M.) ml kg-1 min-1, which occurred at a mean speed of 8.0 ms-1, with the treadmill set at a slope of 10%. At VO2max the mean HR was 222 +/- 7 beats min-1. Maximal Q was 534 +/- 22 ml kg-1 min-1 and mean SV was 2.4 +/- 0.1 ml kg-1. Mean C(a-v)O2 was 24.5 +/- 1.2 ml 100 ml-1 blood. Linear relationships were found between VO2 and work rate, HR, Q and C(a-v)O2. Stroke volume did not increase significantly during exercise. Increasing metabolic rate during exercise was associated with linear increases in arterial haemoglobin concentration and oxygen content, and decreases in arterial oxygen partial pressures and haemoglobin saturation. We conclude that the relatively high VO2max in the detrained Thoroughbred racehorse is dependent on the generation of a large C(a-v)O2, despite development of hypoxaemia and haemoglobin desaturation, during strenuous exercise.