The study aimed to assess the efficacy of high-intensity interval-training (HIIT) compared to moderate-intensity continuous training (MICT) for 12 weeks on a treadmill on maximal aerobic potency, aerobic resistance, respiratory and heart rate at rest, left ventricle structure and function in dogs. The study design was a blinded controlled clinical trial. Thirty untrained dogs between 12 and 84 months were randomly distributed in the groups: HIIT (n=11), MICT (n=9); and control (n=10). Dogs with cardiovascular, neuromuscular, orthopaedic conditions, body condition score over 7, or any abnormalities that impaired exercise performance were excluded. Before and after the programme, dogs performed an incremental exercise test to determine the maximal velocity (Vmax) and velocity at lactate threshold (VLT). Respiratory and heart rates and left ventricular chambers and systolic function at rest...