The effect of electric stimulation of the sympathetic cord in the upper thoracic level on the middle cerebral artery blood flow velocity (VMCA) in humans was examined using transcranial Doppler sonography monitoring during surgery for palmar hyperhidrosis. Sympathetic stimulation resulted in marked and rapid increases of VMCAThe responses were preceded by prompt increases of mean arterial blood pressure (MABP) and heart rate (HR). Division of the sympathetic cord cranially or caudally to the stimulation site partially reduced the VMCA MABP and HR responses. Both these operations reduced sympathetic pathways to the heart as reflected by a decrease in HR and MABP. The integrity of the sympathetic pathway from the stimulation site through the superior cervical ganglion and the carotid plexus was not a prerequisite for a MCAresponse. Our data suggest that the MCA increase mainly results from stimulation of the heart and the cardiovascular system, resulting in marked increases of blood pressure and heart rate.