It was observed through video-recordings that crib-biting horses perform the stereotypy for 10.4 to 64.7 % of their stabling time. Crib-biting and non-crib-biting horses (controls) were exposed to an arousal-inducing stimulus in order to study the similarities and differences in their response. For the first two days of the experiment, the horses received food from a special bucket. On day three, the food bucket was presented but the horses were not allowed to feed. During and after presentation of the food stimulus, arousal behaviour and crib-biting intensity were studied, as was plasma cortisol concentration, heart rate at rest, and heart rate variability at rest. Heart rate and arousal behaviour in crib-biters and in controls increased in response to the stimulus, but crib-biting frequency decreased. Upon examination of heart rate variability at rest, it was determined that crib-biters had a lower vagral tone (high frequency component) and a higher sympathetic tone (low frequency component) than controls. This lower basal parasympathetic activity may be indicating why crib-biting horses showed neither a significant increase in the high frequency component during presentation of the food stimulus, nor decrease in the high frequency component after presentation of the stimulus, when compared with control horses. Therefore, there may be differences in the tuning of the autonomous nervous system and in the stress reactivity of crib-biting and non-crib-biting horses. The experimental results suggest that crib-biting horses are more stress sensitive than non-crib-biting horses, and they are also physiologically and psychologically less flexible than the non-crib-biting horses.