Nervous connections between the trigeminal ganglia and cerebral blood vessels have recently been identified in experimental animals and have been termed the trigeminovascular system. Existence of this system in humans is inferential. Trigeminovascular neurons and their peripheral unmyelinated nerve fibers contain the neurotransmitter peptide substance P. Most newly synthesized substance P is transported from ganglion cell bodies to afferent nerve fibers, where depolarization-induced release of neurotransmitter into the wall of the cerebral blood vessel occurs. Substance P dilates pial arteries, increases vascular permeability, and activates cells that participate in the inflammatory response. The relationship of trigeminovascular fibers to the pathogenesis of vascular head pain sheds light on possible mechanisms of migraine and other central nervous system conditions associated with headache and inflammation.