A quantitative study of responses of pial precapillary vessels to changes in arterial blood carbon dioxide tension was made in anesthetized cats. All vessels examined ranging in diameter from 13 to 90μ responded to hypercapnia with increases in diameter and to hypocapnia with decreases in diameter. The onset of the response to hypercapnia was earlier, its speed was faster, and its magnitude greater with higher concentrations of CO2. In response to the same increase in arterial blood PCO2, smaller vessels (13 to 40μ) increased in diameter by 47.7 ± 10.8% of the control value while the increase in diameter of larger vessels (41 to 90μ) was significantly lower and equal to 29.5 ± 6.3% of the control value. In the steady state the relation between pial vessel diameter expressed as percent of the control value (Y) and arterial blood PCO2 (X) was given by the equation Y = 53.07 + 1.587 X – 0.00715 X2. The relatively large magnitude of the response of pial arteries and arterioles to CO2 and the fact that it was dependent on vessel size show that there must be significant readjustments in the pressure gradients in these vessels when CO2 tension is changed.