With the intraarterial 133Xenon injection method, regional cerebral blood flow (rCBF) was measured from 35 regions of a cerebral hemisphere. The effect of a constant infusion of serotonin (10-33 microg/min) for 2-5 min into the internal carotid artery was studied eight times in five patients. During rest, mean rCBF was 43.5 ml/100g/min as compared to 42.6 ml/100g/min during serotonin infusion. The latter value was altered to 42.8 ml/100g/min if correction for altered PaCO2 was carried out. In neither instance was the difference significant. It is concluded that circulating serotonin is unlikely to cause the cerebrovascular alterations associated with classical migraine attacks.