Total plasma free fatty acids (FFAs), platelet serotonin content and plasma stearic, palmitic, oleic and linoleic acids were estimated in 10 migrainous patients before, during and after a migraine attack. Total and individual plasma FFA levels rose and platelet serotonin fell in most patients. Comparison of the pre-headache and headache mean values showed that of the FFAs linoleic acid rises most during headache. 10 non-migrainous controls had platelet serotonin content estimated before and after the ingestion of 20g linoleic acid. All showed a significant fall in platelet serotonin in the post-ingestion period. It is shown that linoleic acid releases platelet serotonin in vitro, and this study suggests that it has the same action in vivo. Further, it is the precursor of all prostaglandins in the body and its marked elevation during migraine may serve as a source of increased prostaglandin E1 (PGE1) synthesis. It is suggested that linoleic acid plays an important role in the biochemical process of the migraine attack, acting both as a serotonin releasing factor and a source of PGF1, the vasodilating action of which can aggravate the clinical symptoms of migraine.