Internal and external carotid blood flow was measured simultaneously by the use of a recently described electromagnetic flowmeter technique. Intracarotid prostaglandin E1 (PGE1) markedly increased external carotid flow and reduced internal carotid flow, possibly because of “steal” from the internal carotid artery. lntracarotid PGF2a constricted both the internal and external carotid artery, whereas PGF1a had a minimal constrictor effect only on the external carotid artery. Based on the experimental findings, it is hypothesized that the characteristic cranial flow changes of migraine headache may be caused by a single biochemical agent that has a differential effect on external and internal carotid flow to cause simultaneous dilation of the external carotid vasculature and steal of blood from the internal carotid system.