We hypothesised that exercise in a hot and humid environment would affect echocardiography; indices of cardiac dimensions and funtion more than exercise in a cool envirmonment. M-mode echocardiograms were obtained in 5 clinically normal horses before, and at 0, 10, 30 and 60 min after a treadmill exercise test. The test was performed in a controlled environment at 20°C/40% relative humidity (RH) and again at 30°C/80% RH. All the horses completed the exercise test in cool conditions, but only one horse completed the exercise test in the hot/humid environment. The relaxation time index (RTI) correlated significantly and inversely with heart rate (HR). Heart rate was increased and RTI was decreased immediately after exercise in cool conditions. HR recovery appeared to be prolonged following exercise in hot/humid conditions. Post exercise RTI was significantly decreased in hot/humid environment. In hot/humid conditions, the left ventricular internal dimensions measured in diastole and systole (LVIDd, LVIDs) were reduced following exercise and continued to fall throughout the 60 min after exercise. The left ventricular fractional shortening (FS%) was lower than resting levels immediately after exercise in hot/humid conditions. Significant decreases were found in LVIDs and LVIDd, which may reflect a reduction in preload arising from dehydration or re-distribution of blood throughout the recoveryperiod following exercise in the hot/humid environment. It was concluded that echocardiography was a useful tool in assessing cardiac responses to exercise in different climates.