The inhibitory effect of Separan AP-30, an anionic polyacrylamide, on atherosclerotic plaque formation in aortas of rabbits on a high (2%) cholesterol diet was tested over a period extending from 37 to 170 days. Atherogenesis was quantified morphometrically by application of a computerassisted image analysis of histologic cross sections of the aorta. The area of vessel wall-atheroma interface, fraction of lumen occluded, and other indexes of atherogenesis were measured in each of 26 segments of aorta excised from the animals, half of which were administered injections (intravenous) of Separan three times a week. Regression analysis of the morphometric data indicates that the polyelectrolyte exerts a powerful antiatherogenic effect in all regions of the aorta, inhibiting the formation of plaque mass to less than half in the aortic arch and about one-fifth in the descending aorta as compared with the aortic plaque masses in untreated rabbits. Results are compatible with the suggestion that a novel hemodynamic principle in vivo, polymer drag reduction, might be effectively applied against atherosclerosis. Circulation 75, No. 3, 627-635, 1987. THE ADDITION of certain linear macropolymers to flow can under certain conditions greatly reduce frictional resistance by polymer drag reduction, an effect also known among hydrodynamicists as the "Toms phenomenon." Flow can thus be increased by threefold or more without alteration of the driving pressure.'-3The effect occurs only in the presence of disturbed or turbulent flow, and it is associated with a laminarization of the flow. This phenomen has also been observed in pipe blood flow with at least four different drag-reducing polymers, 7 including the anionic polyacrylamide Separan AP-30.Application of the polymer drag-reduction principle to blood flow in vivo was first attempted independently From the