Central excitatory circuits could be involved in the pathophysiology of pain; particularly, the genesis of chronic pain. The "second pain" is the sensation that follows the initial pain after an appropriate nociceptive stimulus. The second pain is amplified by repeating the stimulus after brief intervals (temporal summation). This phenomenon is the psychophysical correlate of the excitatory pain circuits. The temporal summation of the second pain was evaluated in four groups of subjects; one group affected by migraine without aura, one by episodic tension headache, one by chronic daily headache, and a group of healthy subjects. A percutaneous electrical shock was used as the nociceptive stimulus. The intensity of the second pain was significantly greater in the group of patients with chronic headache in comparison with the other groups. The patients with chronic headache were subdivided into three groups on the basis of their clinical history: a group with transformed migraine; a group with chronic headache ab initio, a form related to the first one; (both groups suffered from chronic daily headache with a frequent superimposition of episodes of migraine attacks) and the third group consisted of patients with chronic tension headache. The temporal summation of the second pain was altered in the first two groups. The patients with chronic migraine abused ergotamine given as a symptomatic drug. Those who were able to discontinue this drug were retested and reported a decrease of the second pain in comparison to the previous measurements. The results of the present study indicate that central excitatory circuits could be involved in the mechanism leading to the development of chronic daily headache.