Abstract This study was undertaken to investigate influences of a new vasodilator, YC-93, a derivative of 1,4-dihydropyridine, on cerebral circulation and cerebrovascular CO 2 reactivity. Cerebrocortical Po 2, cerebrocortical Pco 2, cerebrocortical blood flow and arterial blood pressure were continuously recorded by means of a Po 2 electrode, a Pco 2 electrode and a plate-type crossed thermocouple flowmeter placed on an exposed pial surface of the cat brain. The changes in each parameter induced by YC-93 were compared with those induced by papaverine hydrochloride. The effects of 5% CO 2 inhalation were compared before and after the intravenous injection of YC-93. YC-93 (0.01 mg/kg) showed a more marked and longer-lasting hypotensive effect that papaverine (1 mg/kg) and yet produced a significant increase in cerebrocortical Po 2. After the administration of YC-93, the degree of the increase in cerebrocortical Po 2 during 5% CO 2 inhalation was reduced significantly in comparison with that before the administration. The above data indicates that YC-93 has a vasodilating effect on cerebral blood vessels and that the drug causes a decrease in cerebrovascular CO 2 reactivity.