Fluorimetric estimations of total 5HT in blood of 11 migrainous patients during an untreated migraine attack were made. Total blood 5HT dropped during a migraine attack to about 68% of its former level returning to preheadache values by the time the pain ceased. Mean values for total blood serotonin (ng/ml) were 65.1 at the beginning of a migraine attack, 57.9 at the peak, 61.0 at the end, and 91.2 in the headache free period (p<0.05). No correlation between the intensity of the headache and the concentration of 5HT during a migraine attack was found (p>0.5). The influence of some drugs upon fluorescence in blood of migraineurs was tested.