A method for the continuous monitoring of the electrocardiogram (ECG) of horses at rest and during exercise was evaluated in five healthy horses. The horses spent 24 hours in a loose box and performed a standardised exercise test on a treadmill during this period. Two bipolar leads were recorded, lead 1 between the xiphoid and the left withers, and lead 2 between the left ventral thorax and the left dorsal thorax, using disposable electrodes. The quality of the ECGs enabled both an 'operator-controlled' and an automatic analysis to be made. Lead 1 was the most stable during exercise and lead 2 during rest in the loose box. The heart rate and occurrence of arrhythmias were assessed over 22 hours. During the boxed period the heart rate varied from 31 +/- 2 to 138 +/- 11 beats per minute (bpm) and during exercise it varied from 79 +/- 8 to 177 +/- 12 bpm. All the horses had sinus arrhythmias, sinus tachycardias and second degree atrioventricular blocks. One horse developed a ventricular escape rhythm.