Six Dutch warmblood horses, ridden by an experienced rider, cantered 3320 m at a speed adapted to the individual horse's anaerobic threshold, under four different conditions: 1, on a firm shell track with the speed varying from 6.5 to 9.4 m/s; 2, on a horizontal treadmill at the same speeds and for the same duration as in test 1; 3, on a horizontal treadmill for the same duration as in tests 1 and 2, but at a 10 per cent higher speed; and 4, on a treadmill at the same speed and for the same duration as in test 1, but at an inclination of 1 to 2 per cent adjusted individually to obtain heart rates similar to those recorded in test 1. On the basis of the heart rates and plasma lactate concentrations, the workload of the horses was significantly greater in the normal exercise test (1) than in the first treadmill test (2). Increasing the speed of the test on the horizontal treadmill by 10 per cent (test 3) resulted in heart rates and plasma lactate concentrations similar to those recorded in test 1. Inclining the treadmill on the basis of the heart rates achieved in test 1 had a less consistent effect than increasing the speed, but imitated a normal exercise test better than the horizontal treadmill test 2.