Somatosensory-evoked potentials (SEPs) after median nerve stimulation were studied in 34 patients with common migraine, in 30 patients with muscle-contraction headache, and in 10 cluster headache patients. The SEPs were registered before and after histamine administration. Latency values in common migraineurs showed no variation when compared with those in controls. Although not statistically significant, the N1-P2 amplitude was increased in 14 (41.1%) of these patients after histamine stimulation. No changes were observed in muscle-contraction headache patients either with or without histamine administration. In all cluster headache patients, the N1-P2 amplitude decreased after histamine stimulation. These results are discussed in the light of current hypotheses concerning the pathophysiologic mechanisms of headache.