Entry of 125I-labelled low density lipoprotein ([125I]LDL) into the aortic intima was studied over 6 hours in normotenisve and hypertensive rabbits fed a 1% cholesterol diet for 9 and 4 weeks respectively. Studies were also made in hypertensive and normotensive cholesterol-fed rabbits in which blood pressure was reduced acutely with parenteral hydralazine. In all groups the entry of E1125I]LDL was greatest in the aortic arch and significantly less in both the descending thoracic and abdominal regions. Lipoprotein entry into the aorta of cholesterol-fed rabbits was increased some 10-fold over the corresponding value previously found in rabbits fed a normal diet [1]. This increase was due to increased vascular permeability as well as to increased plasma LDL concentration. The hypertensive cholesterol-fed rabbits did not show significantly greater entry of [125I]LDL than the normotensive cholesterol-fed rabbits. Comparison of the rate of LDL entry over 6 house and the quanitity of cholesterol accumulated in the aortic segments over the period of cholesterol feeding indicated that lipoprotein fractions other than LDL must contribute singificant amounts of cholesterol to the developing lesion. The finding that LDL entry paralledled accumulation during cholesterol feeding, together with the finding that acute reversal of hypertension did not reduce the entry of [125I] LDL suggest that mechanisms other than increased filtration of plasma low density lipoprotein contribute significantly to the accelerated development of atherosclerosis in hypertension.