PURPOSE: This study aimed to detect potential differences in heart-rate variability (HRV) during a moderate-intensity intermittent exercise in the heat among physically active young (25.8 +/- 1.9 years), middle-aged (43.5 +/- 2.8 years), and older (62.9 +/- 3.7 years) men. METHODS: Thirty-three participants (11/group) performed four successive bouts of 15-min cycling at a moderate fixed rate of metabolic heat production of ~ 400 W; each separated by a 15-min recovery with 1 h of final recovery in a hot and dry environment (35 degrees C, 20% relative humidity). Twelve HRV indices were computed that have been commonly described in the literature, and characterized various domains of the variability and complexity of heart rate. RESULTS: Cardiac autonomic regulation during intermittent exercise in the heat, as well as during pre-exercise rest and recovery was significantly affected by age, as changes were observed among the three different aged groups in five indices (p </= 0.05). Similarly, time influenced cardiac autonomic regulation as three indices showed changes across time (p </= 0.05) during intermittent exercise, whilst five indices displayed significant changes (p </= 0.05) during rest and recovery in the heat. CONCLUSIONS: This study supports that moderate-intensity intermittent exercise in the heat is associated with significant cardiac autonomic dysregulation in older men, as compared to young and middle-aged men, yet it highlights the importance of developing preventative health strategies for heat-related illness in aged individuals.