Horses’ reactivity was scored using four different methods. Their responses to: isolation from group mates, exposure to an unfamiliar object, passing an unfamiliar object down a hallway was each assigned a subjective emotional score. During each test, the behaviour of the horse was videotaped and the heart rate was recorded. In order for a test to be considered valid, there had to be a demonstrable reaction to the isolation or novel object in terms of the horses’ behaviour or heart rate between horses that were tranquilized and those that received a sham tranquilization. The change in behaviour had to be observed in a minimum of 75% of the trials. There was not a difference in the heart rate of tranquilized and control horses that were passed to a novel objects has they moved down the hallway. As well, during the test only 52% of the trials resulted in the horses exhibiting a change in their behaviour. Therefore, passing by an unfamiliar object while walking down a hallway is not an effective technique at assessing horses’ reactivity. Isolation and an unfamiliar object were both effective stimuli to assess a horses’ reactivity. The most accurate method of assessing the reactivity was the heart rate response. In terms of behaviour, the most reliable indicator was walking when exposed to an unfamiliar object and the number of times a horse defecated in response to isolation. Based upon the heart rate response, isolation and exposure to an unfamiliar object resulted in similar physiological responses and similar reactivity scores. However, the behaviour measurement rated the reactivity of the horses differently. The heart rate and behaviour of horses exposed to an unfamiliar object had comparable reactivity scores, while isolation created some variation between the reactivity score assigned based upon the heart rate or behavioural response. Thus, scoring a horse’s reactivity is more accurate when using the response to an unfamiliar object. The subjective emotion scores were moderately correlated with the heart rate and isolation and with walking scores, in response to an unfamiliar object. The subjective emotionality score were not as accurate as the heart rate and behaviour response to an unfamiliar object. The most accurate way to assess the reactivity of a horses is to used both physiological and behavioural measurements, or by using multiple behavioural measurements.