Increasing performance work in the horse is often associated with a reduction in the forage ration and an increase in concentrates, usually in the form of high-starch cereal grains. This type of diet has been associated with stereotypic behaviours and health problems. High-starch diets are also associated with a high glycaemic response, which has been hypothesised to cause increased reactivity in horses. The aim of this project, therefore, was to evaluate responses of horses to novel stimuli and handling tests when fed a high-starch or a high-fibre diet.A cross-over design study incorporating 2×28day periods was used to evaluate heart rates and behavioural responses during novel stimuli and handling tests when horses were fed a high-starch or a high-fibre diet. All measures were taken over five equally spaced sample points for each period. Data were initially assessed using factor analysis (Varimax rotation, SPSS) in order to produce combined traits of behavioural and heart rate measurements. A Generalised Linear Mixed Model (Genstat® edition 15) was used to analyse the effects of diet on mean data (over the five samples) for each factor as well as for all individual variables. In the handling test, factor 3 (heart rate reactivity) showed a significant relationship to diet (F1,3=27.77, p<0.01). The individual handling test variables of maximum and average heart rates both showed a significant increase when horses were fed the high-starch diet compared to the high-fibre diet (F1,3=10.84, p=0.017 and F1,3=14.42, p=0.01). In the novel test factor 3 (eating reactivity) showed a significant relationship to diet (F1,3=8.02, p=0.03) with horses demonstrating more interrupted eating when they were fed the high-starch diet compared to the high-fibre diet. The individual novel test variables showed that when horses were fed the high-starch diets their maximum heart rates were significantly higher (F1,3=15.31, p<0.01) compared to the high-fibre diet. The results demonstrate that the high-starch diet had some effect on the reactivity and handling behaviour of horses, in particular their heart rate reactivity.