The objective of this experiment was to compare the physiological changes that occurred in horses competing in the cross-country portion of Training, Preliminary and Intermediate horse trials. The hypothesis was that temperature (T), heart rate (HR), respiratory rate (RR), and on-site acid–base and electrolyte monitoring would allow differentiation between difficulty levels in horse trials. Sixteen client-owned horses competing at the Trojan-Horse Spring Horse Trials in Cave Creek, Arizona, USA, were studied. T, HR, RR and anaerobic lithium-heparinized jugular venous samples were obtained before, and 1–2 min after, cross-country exercise. Samples were analysed for blood gases (body temperature-corrected) and concentrations of sodium ([Na+]), potassium ([K+]), chloride ([Cl−]), ionized calcium ([Ca2+]i), blood urea nitrogen ([BUN]) and glucose ([GLC]) using a point-of-care analyser. Results were compared using analysis of variance with the level of significance set at P<0.05. In all groups, exercise increased T, HR, RR, packed cell volume and haemoglobin concentration, and decreased partial pressure of CO2 (PCO2), [total CO2 concentration], bicarbonate concentration ([HCO3−]) and [Ca2+]i (P<0.05). Post-exercise RR was higher, and pH, PCO2, [tCO2], [HCO3−] and base excess were lower, in Intermediate and sometimes Preliminary horses compared with Training horses (P<0.05). It was concluded that horse trials caused milder, but otherwise similar, physical, blood gas and electrolyte changes to those observed in complete three-day events. Physical and point-of-care plasma acid–base and electrolyte monitoring allowed differentiation between horses competing at various levels of difficulty in a horse trial.