The effects of ergot alkaloids on field potentials and unit responses produced in the upper cervical spinal cord by stimulation of the superior sagittal sinus (SSS) were examined in 57 anesthetized cats. Electrical stimulation of the SSS produced field potentials and single-unit responses at latencies of 5-20 ms. Field potentials were abolished by section of the first division of the trigeminal nerve but were unaffected or increased by section of the upper cervical nerves. Field potentials were reduced or abolished by intravenous injection of ergotamine or dihydroergotamine (DHE). The evoked response of 41 units (34.4%) were suppressed by either i.v. or iontophoretic administration of ergotamine, DHE or ergometrine. The results suggest that ergot alkaloids exert an effect at a spinal cord relay centre which receives trigeminally mediated input from cranial blood vessels.