Myocardial mapping in humans has been widely studied and applied to understand heart disease, facilitate early diagnosis, and determine therapeutic targets; however, the reproducibility, repeatability, and protocol-dependent differences in myocardial mapping in dogs remain unknown, which limits its application in dogs. This study investigated the reproducibility and test-retest repeatability of myocardial mapping in dogs and evaluated the differences according to slice, segment, and sequence. Precontrast T1 (native T1), T2 (T2), and T2* relaxation time (T2*), and extracellular volume (ECV) were measured at the base, midventricle, and apex of the left ventricle in six healthy beagles. To compare the sequences, the saturation recovery-based (SMART1) and inversion recovery-based (MOLLI) sequences were used for native T1 and ECV mapping. The intraclass correlation coefficient was measured to evaluate reproducibility and repeatability using the coefficient of variation and Bland-Altman analysis. All parameters showed good to excellent intra- and interobserver reproducibility and test-retest repeatability. The apex slice showed the lowest repeatability among the slices, whereas ECV had the lowest repeatability among the parameters. Native T1, ECV, and T2* did not differ according to slice, but T2 significantly increased from the base to the apex. Native T1 was significantly higher in SMART1 than in MOLLI, whereas ECV did not differ between the two sequences. Our results suggest that myocardial mapping is applicable in dogs with high reproducibility and repeatability, although slice and sequence differences should be considered. This study can serve as a guide for myocardial mapping studies in dogs with heart disease., (© 2024 The Authors. Veterinary Radiology & Ultrasound published by Wiley Periodicals LLC on behalf of American College of Veterinary Radiology.)