This dissertation focuses on the effect of breeding on the myocardium of stallions using cardiac troponin I (cTnI) as the preferred blood biomarker for detection of myocardial cell injury. Haematological, echocardiographic and heart rate (HR) parameters are also reported. Fourteen clinically healthy phantom-trained stallions were assessed. Cardiac troponin I concentration was determined pre-semen collection and at 4, 6, 12 and 24 hours following semen collection. Predictors that were measured included mean HR during each stage of semen collection, maximum HR, area under the curve for HR and fractional shortening (FS). Pre-semen collection cTnI concentrations were within reported reference ranges for all stallions. Following semen collection, cTnI concentration was significantly increased at 4 and 6 hours post-semen collection. Results also suggest that the HR of stallions peak during mounting of the phantom. Five stallions failed to show an expected increase in FS immediately post-semen collection. No association was identified between the measured predictors and increased cTnI concentration. This study concluded that cTnI concentration may increase in stallions following semen collection although the clinical significance of this observed increase in healthy stallions is unclear without histopathological evaluation of the myocardium. No conclusion regarding FS immediately post-semen collection could be drawn from results in this study. The research reported in this dissertation can serve as a reference for future studies investigating cTnI concentrations in stallions.