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In heart transplant (HTx) recipients, there has been reluctance to recommend high-intensity interval training (HIIT) due to denervation and chronotropic impairment of the heart. We compared the effects of 12 weeks' HIIT versus continued moderate exercise (CON) on exercise capacity and chronotropic response in stable HTx recipients >12 months after transplantation in a randomized crossover trial. The study was completed by 16 HTx recipients (mean age 52 years, 75% males). Baseline peak oxygen uptake (VO 2peak ) was 22.9 mL/kg/min. HIIT increased VO 2peak by 4.9 AE 2.7 mL/min/kg (17%) and CON by 2.6 AE 2.2 mL/ kg/min (10%) (significantly higher in HIIT; p < 0.001). During HIIT, systolic blood pressure decreased significantly (p ¼ 0.037) with no significant change in CON (p ¼ 0.241; between group difference p ¼ 0.027). Peak heart rate (HR peak ) increased significantly by 4.3 beats per minute (p ¼ 0.014) after HIIT with no significant change in CON (p ¼ 0.34; between group difference p ¼ 0.027). Heart rate recovery (HR recovery ) improved in both groups with a trend toward greater improvement after HIIT. The 5-month washout showed a significant loss of improvement. HIIT was well tolerated, had a superior effect on oxygen uptake, and led to an unexpected increase in HR peak accompanied by a faster HR recovery . This indicates that the benefits of HIIT are partly a result of improved chronotropic response.Abbreviations: absolute VO 2peak , peak oxygen uptake (mL/min); bpm, beats per minute; CON, continued moderate exercise; DBP, diastolic blood pressure; HIIT, high-intensity interval training; HR, heart rate; HR peak , peak heart rate; HR recovery , heart rate recovery; HR reserve , heart rate reserve; HR rest , resting heart rate; HTx, heart transplant; RER, respiratory exchange ratio; SBP, systolic blood pressure; SET, strength and endurance training; VO 2peak , peak oxygen uptake (mL/min/kg)