INTRODUCTION: Aerobic exercise attenuates age-related declines in resting cardiovagal modulation, as measured by heart-rate variability (HRV) and cardiac baroreflex sensitivity (BRS), which could reduce cardiovascular events and all-cause mortality risk. However, endurance exercise intervention studies in older adults produced equivocal findings. In this respect, the acute exercise model is advantageous as it allows for greater experimental control of exercise and cofounding variables, and may be useful in investigating mechanisms of the exercise response. However, whether ageing alters HRV and BRS responses to acute exercise of different intensities remains unknown. PURPOSE: We compared the response pattern of HRV and BRS at 10 and 60 min following an acute bout of high-intensity interval exercise (HIIE) and moderate-intensity continuous exercise (MICE) in physically active young and older adults. METHODS: Twelve young (20-40 years) and older (57-76 years) healthy and physically active male adults performed an isocaloric acute bout of HIIE, MICE, or a non-exercise condition in a randomized order. Exercise intervention intensity was defined by the maximum oxygen uptake determined during a maximal cardiopulmonary exercise test. HRV time and frequency domain indices and BRS measured at rest, 10 and 60-min following exercise were analysed offline over 2-min time-bins using a software built-in Matlab. Beat-by-beat systolic blood pressure and R-R intervals were recorded in a supine position using finger plethysmography and 5-lead ECG, respectively. BRS was estimated by the spontaneous sequence method, and the time-frequency domain analysis was conducted using the Daubechy-12 discrete wavelet algorithm. Pre-and-post condition changes in outcomes were analysed with linear mixed models. RESULTS: Ln-standard deviation of normal-to-normal intervals (d= -0.53; 95% CI: -0.77 to – 0.30 ms, p<0.001), Ln-root mean square of successive differences (d= -0.85: 95% CI: -1.09 to -0.61 ms, p<0.001), Ln-high-frequency power (d= -1.60; 95% CI: -2.11 to -1.10 ms2; p<0.001), and BRS (d= -6.28; 95% CI: -8.91 to -3.64 ms/mmHg,  p<0.001) decreased following HIIE in young and older adults, but not in MICE (Figure 1). Indices returned to baseline 60 min into recovery. Overall group differences suggested that older adults had lower Ln-root mean square of successive differences (d= – 0.54; 95% CI: -0.93 to -0.15 ms, p=0.009) and Ln-high-frequency power (d= -1.16; 95% CI: – 1.90 to -0.42 ms2, p=0.004), while BRS between young and older adults was not different (d= -2.98; 95% CI: -5.96 to 0.00 ms/mmHg; p=0.05).  CONCLUSION: We found no evidence of age-associated response patterns in HRV or BRS to a single bout of HIIE or MICE in physically active participants. HIIE reduced cardiovagal modulation in physically active young and older adults, returning to baseline values 60 min into recovery. This might increase the likelihood of cardiovagal supercompensation towards resting vagal predominance on the long run without increasing the risk for acute cardiovascular events in physically active young and older adults given the fast cardiovagal reactivation observed. ETHICS: This study was aligned with the Declaration of Helsinki and approved by the Ethical Review Board of the Faculdade de Motricidade Humana, Universidade de Lisboa (10/2020). Informed consent was obtained from all studied participants.