The objective of this study was to investigate in Thoroughbreds, the relation between the development of the heart at prerace period and the velocity at maximum heart rate (VHRmax) between the 24th and 28th month before starting to race. Twenty-one Thoroughbreds (11 female, 10 male) were used in this study. The echocardiographic data were obtained from the short-axis view in guided M-mode, at 4-month intervals until the 24th month after birth. The VHRmax was measured monthly from the 24th to the 28th month by E-Trakka. Linear regression analyses were performed between echocardiographic parameters and VHRmax values at 24 and 28 months after birth. The results indicated that left ventricular internal diameter in diastole at 12 months (LVIDD12) tended to be negatively correlated with VHRmax measured at the 24th month (P = .098). However, ejection fraction (EF12) (P = .061) and fractional shortening (FS12) (P = .056) values tended to be positively correlated with VHRmax measured at the 24th month (VHRmax(24)). There was a tendency toward negative correlation between interventricular septal thickening percentage (IVST%) measured 1 month after birth and VHRmax measured at the 28th month (VHRmax(28)) (P = .079). In contrast, there were positive correlations between mean wall thickness of 4 months (MWT4), relative wall thickness of 4 months (RWT4), and VHRmax28 P < .05). EF16 and FS16 tended to be correlated with VHRmax(28). Furthermore, EF and FS parameters measured in 20 months after birth (EF20 and FS20) were correlated with VHRmax28 (P < .05). There was a tendency toward a positive correlation between the left ventricular muscle mass of horses at 24 months of age (LVM24) and VHRmax(28) (P = .097). In addition, mean wall thickness of 24 (MWT24) months and relative wall thickness of 24 months (RWT24) were significantly correlated with VHRmax(28) (P < .05). In this study, we observed a relationship between some echocardiographic parameters measured during developmental period and VHRmax measured during prerace training period. Because VHRmax is related to athletic potential, echocardiographic parameters measured at early ages may be used as selection criteria. (C) 2013 Elsevier Inc. All rights reserved.