Abstract Objective—To assess whether physical training induces cardiac hypertrophy and valvular regurgitation in maturing Standardbred trotters and to establish a prediction model for the size of the left ventricle. Design—Longitudinal observational study. Animals—53 Standardbred trotters. Procedures—Each horse underwent 2-D, M-mode, and color flow Doppler echocardiography at 5.5 years of age; previously, each horse had been examined at 2, 2.5, 3, and 3.5 (time of maturity) years of age. Horses were or were not in training or racing for variable periods during the entire assessment period; data for a given horse were assigned to 1 of 2 groups on the basis of the horse's status at the fifth examination (racing [n = 40] or not racing [13]). At each examination, left ventricular (LV) internal diameter in diastole (LVIDd), LV mass, and mean and relative LV wall thicknesses were measured. Prevalence and severity of tricuspid, pulmonary, mitral, and aortic valve regurgitation were determined. Results—During the assessment period, LVIDd, LV mass, and mean LV wall thickness increased; body weight was significantly associated with those variables. Prediction of LV mass was possible when sex and weight were included in the model. Prevalence of valvular regurgitation increased for all valves. An increased risk of development of tricuspid and pulmonary valve regurgitation for horses in racing was observed. Conclusions and Clinical Relevance—The LV mass and prevalence of valvular regurgitation increased (indicative of development of exercise-induced cardiac hypertrophy and valvular regurgitation) in young horses, even during the latter part of the assessment period, when maturity was attained.