The changes of cerebral blood flow velocity (CBFV) and pulsatility index (PI) in anterior cerebral artery were examined by cranial color Doppler ultrasonography while a patient recovered from hypocarbia. As arterial PCO2 was elevated, PI was decreased and color imaging of CBF from basilar artery to anterior cerebral artery were improved in sagittal view. Hypocarbia is considered to induce the constriction of the cerebral vessels and the decrease of CBF in premature infants.