The effects on cerebral circulation and intracranial pressure (ICP) of the carotid sinus nerve stimulation were examined in mongrel cats anesthetized with alpha-chloralose. In 214 trials with stimulation at normal ICP (less than 20mm Hg) in seven cats, mean arterial blood pressure (mABP), heart rate (HR), and ICP decreased significantly (p less than 0.05 in paired Student's t-tests), and the decreased magnitudes for mABP, HR and ICP were 29.9mm Hg, 32 beats/min and 0.9mm Hg, respectively. Correlation was linear between changes in the ICP and mABP (r = 0.61). After spinal ligation at the second cervical segment, bradycardia and depressor responses to the stimulation were observed. However, after vagosympathectomy of the cats with ligated cords, sinus nerve stimulation did not change the ABP, HR, ICP, CBF, or cerebral blood volume. This suggested that carotid sinus baroreceptor reflexes had no direct effect on cerebral vasodilation. When the levels before stimulation of the ICP were increased without evocation of Cushing's response by gradual inflation of a balloon placed extradurally, the ABP and the HR still decreased after stimulation. When Cushing's response was brought about, the responses in ABP and HR caused by the stimulation were not apparent at the peak of arterial hypertension. However, the amplitude of the decrease in the ICP caused by stimulation was enhanced with an increase in the levels of ICP before stimulation. Thus, the arterial baroreflex is preserved at increased levels of ICP without Cushing's response, and works more effectively to cancel the possible increase of ICP that can arise when there is a pulsatile increase in the ABP for one of several reasons.