This study delves into the physiological responses of 20 elite eventing horses to submaximal exercise, specifically examining changes in blood parameters before and after participating in two four-star cross-country rides. The research utilized a mixed model that accounted for plasma volume shifts to evaluate exercise-dependent parameters and compare them against standard reference ranges for resting healthy horses. Key findings include short-term increases in cortisol, T3, T4, and superoxide-dismutase immediately following exercise. Hepatic markers such as bilirubin, glutamate dehydrogenase, and alanine aminotransferase exhibited either short-term or sustained elevations, indicating potential hepatocellular injury. Digestive enzymes and lipid levels showed minor fluctuations, while zinc and iron levels decreased post-exercise. Notably, white blood cell counts increased persistently, suggesting a mild inflammatory response, although serum amyloid A levels remained unchanged. The study highlights the activation of stress-related hormonal axes and antioxidant activity, alongside the mobilization of energy reserves and signs of mild hemolysis and hepatocellular injury. This research underscores the need for further investigation into individual variations among horses in response to exercise.