Tissue oxygenation estimated by near-infrared spectroscopy (NIRS) is a volume-weighted mean of the arterial and venous hemoglobin oxygenation. In vivo validation assumes a fixed arterial-to-venous volume-ratio (AV-ratio). Regulatory cerebro-vascular mechanisms may change the AV-ratio. We used hypotension to investigate the influence of blood volume distribution on cerebral NIRS in a newborn piglet model. Hypotension was induced gradually by inflating a balloon-catheter in the inferior vena cava and the regional tissue oxygenation from NIRS (rStO₂,NIRS) was then compared to a reference (rStO₂,COX) calculated from superior sagittal sinus and aortic blood sample co-oximetry with a fixed AV-ratio. Apparent changes in the AV-ratio and cerebral blood volume (CBV) were also calculated. The mean arterial blood pressure (MABP) range was 14 to 82 mmHg. PaCO₂ and SaO₂ were stable during measurements. rStO₂,NIRS mirrored only 25% (95% Cl: 21% to 28%, p<0.001) of changes in rStO₂,COX. Calculated AV-ratio increased with decreasing MABP (slope:-0.007·mmHg-1, p<0.001). NIRS estimates that CBV decreased with decreasing MABP (slope: 0.008ml/100g/mmHg, p<0.001). Thus, cerebral NIRS oximetry responded poorly to changes in tissue oxygenation during hypotension induced by decreased preload. An increase in the AV-ratio during hypotension due to arterial vasodilation and, possibly, cerebral venous collapse may be a part of the explanation.