Background: Cardiac arrhythmias in exercising horses are the focus of much interest, both in terms of what is considered normal and potential associations with poor performance and sudden cardiac death. One barrier to performing large-scale studies is the lack of an easily applicable device, to allow recording of large numbers of highquality exercising electrocardiograms (ECGs). The Equimetre TM is a new wearable device, which records a single lead ECG, among other parameters. Validation of such wearable devices is essential before further studies are undertaken. Objectives:To evaluate the quality of ECG using the Equimetre TM and compare arrhythmia detection during exercise with the reference Televet TM system. Study design:Prospective blinded clinical study.Methods: Simultaneous ECGs were recorded with both systems in 49 healthy horses during exercise. High-intensity exercise (>40 km/h) was performed by 29 racehorses, and lower-intensity exercise for the remainder of the racehorses and show jumpers.Tracings were excluded if >10% artefact was present (duration of artefact relative to duration of exercise). For included ECGs, the duration of artefact was recorded and compared. ECGs were evaluated using Kubios premium software. Arrhythmia detection (yes/no) and arrhythmia classification (sinus arrhythmia, narrow complex of similar morphology to the sinus complexes or wide complex with a different morphology) were compared using Cohen's Kappa coefficient.Results: Nine Televet TM ECGs and 3 Equimetre TM ECGs were excluded due to artefact >10%. Televet TM ECGs included significantly more artefact during exercise than Equimetre TM ECGs (5% vs. 0.25% P < .001). Arrhythmia analysis was performed on 38 horses' paired ECGs. The kappa coefficient was excellent for arrhythmia detection (K = 0.97) and arrhythmia classification (K = 0.93). Main limitations:Relatively low numbers of horses with arrhythmias (n = 21) were included. The ECG recordings only provided one lead, making arrhythmia classification challenging in some cases.