Knowledge of the competitive demands of different sports or activities is important for designing appropriate training programmes to ensure that animals reach a sufficient level of fitness to reduce the risk of overexertion and injury or illness and to achieve the best possible performance in relation to an individual's genetic potential. Whilst the physiological demands of many equestrian sports have been described, to the best of our knowledge the cardiovascular demands of polo have not. The aims of the present study were therefore to record heart rate during and after competitive polo games in a group of low-goal (non-elite) polo ponies in order to describe the absolute heart rates during play, the relationship of these heart rates to maximal heart rate and the characteristics of a typical chukka in terms of effort. Six low-goal polo ponies were studied during a total of 59 chukkas. Heart rate was monitored continuously before, during and after competition using a commercial heart rate monitor. Maximal heart rate was determined with field and treadmill incremental exercise tests and used to express work intensity in terms of time during play that each ponies heart rate was less than 75% HRmax, between 75 and 90% HRmax and greater than 90% HRmax. Mean maximum heart rate was not different during play or during field and treadmill exercise tests; 215+/-7 (mean +/- s.d.), 211+/-7 and 213+/-2 beats/min, respectively (P>0.05). Mean heart rate for all ponies over all chukkas was 166+/-6 beats/min with a mean chukka duration of 611+/-18 s. Of this time, 44+/-7% of the time was spent below 75% HRmax, 39+/-8% between 75 and 90% HRmax and 17+/-8% of time above 90% HRmax. When only one chukka had been played, there was a good correlation between mean heart rate during play and 3 min recovery heart rate (r = 0.63, P<0.001). Based on these observations, it is proposed that low-goal polo places moderate to high stress on the cardiovascular system.