By grouping patients who had carotid angiograms under unusually carefully monitored conditions it has been shown that hypocapnia is associated with vasodilatation at low blood pressure but not at high blood pressure. The mechanism is discussed in general terms and it is suggested that the hypocapnic vasodilatation may be a response to cerebral hypoxia and may be transmitted via an intracerebral autonomic pathway. Clinical and angiographical diagnoses are given for SO patients.