Carbon dioxide-induced changes in near infrared spectrophotometry recordings were compared with changes in cerebral blood flow estimated by133Xenon clearance (global cerebral blood flow (infinity)) at serial measurements in 24 mechanically ventilated preterm infants (mean gestational age 30.2 wk). In all infants, three measurements were taken at different arterial carbon dioxide tension levels (mean 4.4 kPa, range 2.1-7.8) obtained by adjustment of the ventilator settings. Mean arterial blood pressure changed spontaneously, whereas arterial oxygen tension was kept within normal range. At all wavelengths (904, 845, 805, and 775 nm), the OD increased at higher arterial carbon dioxide tension levels, indicating cerebral vasodilation. This conclusion was supported by conversion of the data to changes in oxygenated and deoxygenated Hb concentration. A parallel increase in cerebral blood volume index and global cerebral blood flow (infinity) was found (P<0.0001). The oxygenation level of cytochrome aa3increased with increases in oxygen delivery (P<0.0001). This observation, however, may have been artifactual due to cross-talk between the oxidized cytochrome aa3and the oxygenated Hb signals, as these signals were closely interrelated in the present experimental design. We suggest that near infrared spectrophotometry may be used for estimation of the cerebral blood volume index/cerebral blood flow-CO2reactivity within a wide range of arterial carbon dioxide tension. Knowledge of the light path length would put this estimation on a quantitative basis.