SYNOPSISThe mean platelet 5‐HT concentration in 24 migrainous patients during headache‐free intervals (580 ± 53.3 ng109platelets, M ± SEM) was similar to that found in 25 healthy persons (550 ± 48.1). A significant fall in platelet 5‐HT level (about 15% p<0.05) was found during migraine attacks as compared with headache‐free intervals. When platelets from blood collected during headache‐free intervals were incubated with platelet‐poor plasma taken from another migrainous patient in attack, the 5‐HT content diminished by 22%. On the other hand, plasma taken in the headache‐free period did not produce a considerable release of platelet 5‐HT. Moreover, plasma from migrainous sufferers in the attack did not elicit 5‐HT release from platelets of healthy persons. These results lend support to the hypothesis that a 5‐HT releasing factor appears in plasma during the migraine attack, and that some platelet abnormality is p