The purpose of the present study is to clarify the effect of stimulation of the reticular formation of the medulla oblongata (MORF) on cerebrovascular tone. Using 32 cats, blood pressure (BP), intracranial pressure (ICP) and cerebral blood volume (CBV) were continuously measured. The animals were divided into 4 groups: G(1), normal ICP; G(2), increased ICP due to subarachnoid hemorrhage; G(3), superior cervical ganglionectomy, and G(4), spinal cord transection at C(2). In all animals, the MORF was stimulated after the dorsomedial nucleus of the hypothalamus and the reticular formation of the midbrain were destroyed. The BP was also increased with an intravenous administration of angiotensin II. The vasomotor index (VI), defined by ΔICP/ΔBP, was calculated and compared between the two situations, i.e., MORF stimulation and angiotensin II administration. BP, ICP and CBV increased simultaneously immediately after MORF stimulation. In G(1), G(2) and G(3), the VI with MORF stimulation was significantly higher than that with angiotensin II. The increase in ICP with MORF stimulation was also observed in G(4), although the BP showed no consistent changes in this group.