The trigeminal ganglion was activated, in humans by thermocoagulation as part of the treatment of trigeminal neuralgia and in cats by electrical stimulation, and blood samples were taken from the external jugular vein for estimates of plasma levels of substance P and calcitonin gene-related peptide (CGRP). In those patients who were noted at the time of coagulation to have flushed there were marked elevations of the local (cranial) levels of both peptides. However, in the nonflushing patients no changes in the peptide levels were observed. Parallel experiments in the cat revealed that the levels of substance P—like and CGRP-like immunoreactivity were increased during electrical stimulation of the trigeminal ganglion. The observation of elevation of substance P—like and CGRP-like immunoreactivity after activation of the nociceptive afferent system of the head provides new insights into a putative role of peptides in the pathophysiology of migraine and cluster headache, and suggests new areas of possible therapeutic intervention.