Although it is generally accepted that the arterial tree stiffens after prolonged exposure to elevated pressure, the sequence of the changes in the mechanical properties of the vessel wall which occur during the early phase of hypertension has not been fully elucidated. The objectives of this study are to identify these changes, document when they occur, and expose the underlying structural alterations which cause the modification of the elastic properties of the wall. To accomplish these goals, during the past year we measured the variation of aortic diameter and length with aortic pressure at weekly intervals for six weeks after the onset of deoxycorticosterone acetate (DOCA) induced hypertension in swine. Aortic diameter and length were measured using an ultrasonic strain gauge. Preliminary results indicated that the response of aortic wall to hypertension is biphasic. The wall first becomes more compliant, then stiffens in response to increased pressure. Currently, we are studying the histological changes in the aortic media in early hypertension. Using quantitative light and electron microscopy technique we will determine whether changes in the amount of connective tissue (collagen and elastin) occur in early hypertension. To determine the anatomic basis for the changes we have observed, the results of this study will be correlated with our previously obtained data on the mechanical properties of the aorta. This study should provide new insight into the role which structural adaptations play in the maintenance of hypertension.