SUMMARY A combined transmission (TEM) and scanning (SEM) electron microscopic study was performed on aortae of deoxycorticosterone-salt (DOC-salt)-treated rats and spontaneously hypertensive rats (SHR) to compare the effects of hypertension as well as its reversal on the aortic intima. To best reproduce the in riro state of the vasculature, rats were perfusioD-fixed at pressures corrected for each individual animal (30 mm Hg below measured systolic pressure). The intimal alterations were focal and thus were best appreciated with the combined use of SEM and TEM. Qualitatively, both models of hypertension showed similar intimal changes, which consisted of subintimal thickening due to an accumulation of both extracellular material and cells. Subendothelial cells with a morphology indicating a blood-borne origin were present simultaneously with cells derived from the vessel wall. The increased subendothelial extracellular material included precipitated plasma proteins, reticulated basement membrane, collagen fibers, and fragments of elastln. Increase in the height of endothelial cells with distortion of nuclear shape was prominent. Withdrawal of DOC-salt combined with low-salt diet for 11 weeks did not result in a discernible regression of these intimal changes despite normalization of blood pressure. We conclude that vascular injury, once induced, may be difficult to reverse and suggest that areas of prior damage may serve as foci for later vascular complications.