Carbon dioxide (CO2) is a potent cerebrovasodilator; even mild changes in CO2 are associated with marked changes in cerebral blood flow (CBF). We measured CBF before and after 5% CO2 inhalation in 19 medicated patients with schizophrenia and 16 normal volunteers. Another group of 16 volunteers had 2 CBF measurements under resting conditions. Although both patients and controls showed marked CBF increase during CO2 inhalation, the CBF response was significantly less in the patients. Change in CBF per mm of CO2 was lower in the patients. The second group of controls did not show significant differences between the 2 resting CBF measurements.