The effect of the serotonin antagonist iprazochrome was stuide d in 30 patients with atypical facial pain and 10 control nonpain volunteers. The aim of the study was to investigate the effect of a single dose of iprazochrome and of its short-term administration in patients with chronic pain. Twelve of the 30 patients reported increased pain, 16 reported no effect, and two reported pain relief for some horus after taking iprazochrome. None of the controls reported any effect from iprazochrome. Four of the 12 patients who reported increased pain also reported increased pain in distant organs, especially in the joints. A hypothesis of the mechanism is presented and a possible use of iprazochrome as a diagnostic tool is suggested.