The vascular reactivity to CO2 inhalation was studied simultaneously in the ipsilateral middle cerebral (MCA) and ophthalmic arteries (OphA) of 10 healthy subjects using 2-vessel transcranial Doppler sonography. Flow velocities were measured simultaneously in both arteries during normo- and hypercapnia. A reactivity index (IxR) was defined in terms of the relative velocity change in response to an increase in end-tidal CO2 pressure of 5 mm Hg. In the MCA IxRwas 21.9 ± 3.6%, whereas in the OphA flow velocity decreased significantly (IxR(= –4.8 ± 2.9%). This latter change may reflect dilation of the ophthalmic artery during hypercapnia. Testing of oculovascular CO2 reactivity is a reliable method that may be of interest for the evaluation and management of patients suffering from ocular ischemic syndromes due to carotid artery obstructi