The purpose of this study was to assess the possible relationship between maximal running speed, serum isoenzyme patterns of creatine kinase (CK) and lactate dehydrogenase (LDH) and echocardiographic indices of left ventricular function. A group of 15 healthy, 3-year-old Maremmano stallions were given a 100 day training programme. At the end of this the animals carried out a maximum speed test and were divided into 2 groups (A and B) according to whether or not they had attained a speed of 15 m/s. Venous blood samples were taken from each horse before exercise (T0), 2 min (T1) and 24 h (T2) after exercise. Total serum activity of CK and LDH was measured and their isoenzyme distribution pattern determined. The day before the speed test echocardiographic examination was carried out at rest to assess the left ventricular function by calculating telediastolic, telesystolic and stroke volume, ejection fraction and stroke index. Statistically significant differences were found for the CK isoenzyme pattern at T2, where Group A showed an increase in the MM fraction (P=0.003) and a decrease in the MB fraction (P = 0.014). These changes were thought to be linked to an increased membrane leakage due to exercise and not to muscle fibre disruption because the CK and LDH total activities remained within the normal range. In Group A there was also greater left ventricular telediastolic volume (P=0.044) and length (P=0.033) at rest as well as a greater stroke index (P = 0.032). We concluded that the evaluation of CK pattern after exercise and of echocardiographic left ventricular function indices at rest made it possible to select for the fastest horses (Group A).