In 55 patients, the blood flow in the carotid and supratrochlear arteries was examined by Doppler sonography during cerebral angiography. In general, a threefold response was observed: an initial phase of turbulence during the injection was followed by a phase of depressed and then of increased arterial flow. The second phase of reduced perfusion which probably is caused by the increased viscosity of the mixture of contrast medium and blood, is more pronounced in the internal carotid artery, whereas the third phase of accelerated flow is more marked in the external carotid artery. In patients with an occlusion of the internal carotid artery, the cerebral blood supply often depends on the collateral pathway via the ophthalmic artery. In these cases, the reduction of the blood flow is specially marked and long-lasting, as we could show. This may contribute to a higher angiographic risk of those cases with an important ophthalmic collateral flow.