The response of cerebral blood volume (CBVR) to a small induced change in arterial carbon dioxide tension was studied by near-infrared spectroscopy in 17 newborn infants born from 26 wk of gestation to term. All 17 infants were undergoing mechanical ventilation but had apparently normal brains. The CBVR per kPa change in arterial carbon dioxide tension within the range 3.9 to 9.6 kPa was calculated from the change in total cerebral Hb concentration ([TCHb]) using the equation: ΔCBV = Δ[TCHb] x 0.89/[H] where [H] is the large vessel Hb concentration. A least-squares regression line with 95% confidence limits was derived for CBVR against gestational age. A highly significant linear increase in CBVR was found: mean CBVR from the regression increased from 0.07 mL·100 g-1·kPa-1at 26 wk to 0.51 mL·100 g-1·kPa-1at 40 wk.