Background and aims: Impaired cerebrovascular reactivity to carbon dioxide (CVRCO2) is an established risk factor for stroke (1) and has been linked with cognitive dysfunction (2). Preliminary evidence suggests that CVRCO2 may be improved through increases in physical activity (3; 4). However, data are scarce and cross-sectional studies are lacking. Methods: Ten untrained (25 + 5 years; 86 + 12 kg; < 150 minutes of aerobic exercise per week; maximal oxygen consumption (VO2MAX), 37 + 5 ml.kg-1.min-1) and 10 trained (23 + 4 years; 79 + 10 kg; > 150 minutes of aerobic exercise per week; VO2MAX, 57 + 8 ml.kg-1.min-1) healthy males participated in the study. CVRCO2 was assessed in response to a 3-minute exposure to hypercapnea (5% CO2). Blood flow velocity in the middle cerebral artery (transcranial Doppler) and end-tidal CO2 (capnography) were continuously recorded throughout the procedure as described previously (5). VO2MAX was assessed via an incremental exercise test to volitional exhaustion (semi-recumbent cycle ergometer) and online expiratory gas exchange. Following confirmation of distribution of normality (Shapiro-W-Wilk Tests), CVRCO2 and VO2MAX data were analysed using independent samples t-tests. The relationship between VO2MAX and CVRCO2 was examined using a Pearson Product Moment correlation. Significance was established at P < 0.05 and data are expressed as mean + SD. Results: By design, the trained participants recorded a higher VO2MAX (P < 0.05). The trained were able to elicit a greater CVRCO2 compared to their untrained counterparts (3.64 + 1.1 vs. 1.87 + 0.8 %/mmHg; P < 0.05) during the hypercapnic challenge (Figure 1). Figure 2 illustrates the positive correlation between VO2MAX and CVRCO2 (r = 0.79; P < 0.05). Conclusion: These findings highlight the cerebrovascular benefits of regular physical activity, which may act as a potential preventative mechanism against cerebrovascular diseases in later life.