Capsaicin, when repeatedly applied to the nasal mucosa of cluster headache patients, has been shown to prevent the occurrence of pain attacks. In order to investigate the mechanism of the drug's action, we evaluated the effect of repeated nasal application of capsaicin on the contents of sensory fibres immunoreactive to substance P and CGRP in the rat nasal mucosa. Further, considering the possible involvement of the cerebral circulation, we verified the effect of a single application of capsaicin on the blood flow velocity of the internal carotid and middle cerebral arteries (of both sides) and the basilar artery, in a group of healthy humans. The measurements were taken using Doppler devices. In order to verify the reproducibility of therapeutic effect of capsaicin, we carried out a 2-year follow-up study on patients affected by cluster headache (17 by episodic form, 8 by chronic form) who responded positively to the first treatment with capsaicin. During this period they were treated again with capsaicin in case of re-occurrence of symptoms. Capsaicin depletes the fibers immunoreactive to substance P and CGRP in the rat nasal mucosa. In the healthy controls, a single application induced vasodilation in the internal carotid, whereas middle cerebral arteries and basilar artery were narrowed. The results of the follow-up study, demonstrates that in 65% of the patients, the beneficial effect of capsaicin was again present when the treatment was repeated. In the chronic patients the therapeutic effect was always transitory (lasting, at maximum one month).(ABSTRACT TRUNCATED AT 250 WORDS)