Objective <p>Cerebral blood flow autoregulation is affected by several physiologic and medical factors. Especially arterial carbon dioxide pressures (PaCO₂) impact cerebral blood flow. Only extensive changes in end-tidal CO₂ have been studied so far. The aim of this study was to evaluate the impact of mild hypo- and hyperventilation on cerebral blood flow as assessed by regional cerebral red blood cell oxygen saturation (rSO₂) in two age groups.</p> Methods <p>Two groups of patients were compared under general anesthesia before the surgical procedure was started: A younger patient group (age &lt; 40 years; YP) and older patients aged &gt; 60 years (OP). Anesthetic management was standardized. In both groups, end-tidal CO₂ was adjusted either to a low-normal value of 35–37 mmHg or a high-normal value of 43–45 mmHg for 15 min each. The sequence of these interventions was randomized. rSO₂ was estimated by near-infrared spectroscopy (NIRS). The primary outcome was defined as the difference in rSO2 between hypo- and hyperventilation between the two age groups.</p> Results <p>A total of 78 patients were included. In both groups, there was a statistically significant difference in rSO2 values after 15 min of hypo- versus hyperventilation. In the YP-group, rSO₂ was 74 ± 4% after 15 min of hypoventilation and decreased to 68 ± 6% during hyperventilation (<i>p</i> &lt; 0.001). In the OP-group, rSO₂ was 71 ± 5% and 65 ± 6%, respectively (<i>p</i> &lt; 0.001). There was no difference concerning changes in comparison of younger and elder patient groups (in both groups, Δ rSO₂ = 6 ± 3%; <i>p</i> = 0.732).</p> Conclusion <p>Even mild hypoventilation increased rSO₂ compared to mild hyperventilation and this difference occurred independent of age.</p>