The purpose of this study was 1) to measure and compare some physiological parameters in horses during a standardised treadmill exercise performed either in temperate atmospheric conditions (TC) (ambient temperature: 15 degrees C; relative humidity: 55%), or in hot and humid conditions (HHC) (ambient temperature: 30 degrees C; relative humidity: 75%) and 2) to follow the recovery of the same horses during 1 h after both tests. Five healthy fit Standardbred horses were investigated twice at 8 days interval, in TC and in HHC. Some measurements were made during a standardised treadmill exercise test (SET), others during a 1 h recovery period. The SET consisted of 8 min warm-up and 8 min exercise. During the SET, respiratory airflow and O2 and CO2 fraction in the respiratory gases were continuously recorded, using 2 ultrasonic pneumotachographs connected to a face mask and a mass spectrometer. Oxygen uptake (VO2), carbon dioxide output, respiratory quotient and expired minute volume (VE) were obtained on a breath-by-breath basis. The maximal values obtained during the highest intensity of the SET and the values obtained 2 min after the end of the test were used for the comparison between both tests. Heart rate (HR) and skin temperature were continuously recorded, using a heart rate recording system and a real time infrared thermographic unit, respectively. After both SETs in TC and HHC, the horses recovered in TC. Heart rate, respiratory frequency (f) and rectal temperature were regularly measured during 1 h after the test. Venous blood was sampled after the completion of the test for biochemical analysis, namely plasma electrolytes and enzymatic activities. Exercising in HHC induced a reduction of the aerobic metabolism to the total energy requirement, i.e. a 20% decrease of VO2 and 55% increase in lactate, as well as a decrease in VE. The increase in rectal and skin temperature, the dehydration and the weight loss were higher in HHC than in TC. During the recovery period the HR and f remained higher up to 30 min after the end of the SET in HHC while the rectal temperature remained higher up to 60 min after the end of this test. Therefore, despite the fact that our horses recovered in TC after exercising in HHC, there were significant differences between their physiological parameters measured during the recovery. This means that the recovery in air-conditioned unit would probably be an insufficient measure to ensure an adequate cooling of the horses and justify consequently the adaptations proposed by the Federation Equestre Internationale (FEI), i.e. the decrease of the distance of the run and the increase of the time devoted to the halt between phases.