Alteration of platelet serotonin in patients with chronic tension-type headache during cold pressor test. Change of 5-hydroxytryptamine (5-HT) concentrations in platelets from patients with chronic tension-type headache (TH) and controls were observed during cold pressor test (CPT). Before cold stimulation, 5-HT concentrations in platelets from patients with TH were significantly lower than those found in controls. One minute after the start of cold stimulation, 5-HT levels in platelets from patients with TH were significantly much lower than in the controls, as control levels rose and TH levels fell. The results show that, under stress, the absorbance of 5-HT into the platelets in patients with TH is reduced. It is suggested that, in patients with TH, there are abnormalities of 5-HT uptake into platelets and factors which cause release of 5-HT from platelets. 