The stainless steel cannula inserting method was modified for application to observe vascular responses to thiopentone in the isolated basilar arteries of the dog and monkey. In the dog, thiopentone (0.01-3 mg) induced a monophasic vasoconstriction in a dose-dependent manner. On the other hand, in the monkey, thiopentone (0.01-3 mg) showed a biphasic vascular response, i.e., an initial vasoconstriction followed by a vasodilatation in a dose-dependent manner. Thiopentone usually produced much more potent vasoconstriction in the dog than that in the monkey, while potassium chloride made little difference of vasoconstriction between the dog and monkey basilar arteries. These findings suggest that thiopentone exerts a direct constrictive effect on cerebral vessels, the actions of which decrease the total cerebral blood volume and the brain bulk, allowing a reduction in intracranial pressure.