An understanding of the factors which produce a “backward” angiogram of the contralateral vertebral artery is important for two reasons: 1. 1. Reverse flow of blood and contrast material in a vertebral-subclavian circuit secondary to proximal vascular obstruction is well documented (2, 6, 14, 18). It would appear, however, that conditions other than proximal contralateral vertebral, subclavian, or innominate artery obstruction as well as basilar artery thrombosis (16) allow reflux to occur, particularly when subclavian or brachial artery injections are made (3, 5, 11, 17). An analysis of the possible causes of this phenomenon by means of in vivo and in vitro laboratory methods has been carried out. 2. 2. The radiological detection of a source of subarachnoid hemorrhage not infrequently requires the thorough opacification of the vertebral-basilar circulation. When reflux of contrast material is adequate in the contralateral artery, a posterior inferior cerebellar artery aneurysm can be excluded by means of unilateral vertebral injection. Hopefully, the present laboratory survey will aid in the development of technics which promote vertebral artery reflux. If this can be done, a second puncture to study the opposite vertebral artery may not be necessary. Objectives The presence and degree of contralateral vertebral artery reflux were evaluated in relation to (a) catheter position—sub-clavian or vertebral, (b) injection pressure, (c) contrast volume, (d) head positionfrontal or oblique, animal supine, (e) hypotension, (f) increased pulse pressurearteriovenous fistulae, aortic regurgitation, (g) changes in pCO2 level, (h) cerebral edema, (i) flow rate in vertebral-basilar artery model, and (j) configuration and plane of vertebral-basilar artery junction in model. Methods The left subclavian artery as well as the cervical course and union of the vertebral arteries in the dog are similar to those in man (Fig. 1) and serve as a test system. Small muscular branches from the vertebral artery form connections via their opposite number in the neck. These permit minimal contralateral vertebral artery filling. Most of the contrast material, however, refluxes at the junction of the vertebral arteries. Catheterization of the left subclavian artery of anesthetized animals was carried out via the femoral route or by exposure of the axillary artery. Catheterization of the left vertebral artery was performed following installation of the catheter in the left subclavian artery after transaortic manipulation of the catheter from the right femoral artery.