Structural arterial adaptation and decompensation were studied in the contralateral untouched kidneys of two-kidney, one clip hypertensive rats 1 to 64 days after constricting one renal artery. Focal necroses of intrarenal arteries were observed as early as 24 hours after starting the experiment. The necroses reached their maximum on Day 8 and thereafter decreased significantly in spite of still increasing blood pressure values. After 8 days the media thickness of the interlobular arteries was significantly increased by more than 30% and remained so until the end of the experiment. Blood pressure levels as measured by tail plethysmography were nearly normal within the first 4 days. The continuous 24-hour blood pressure recording in the conscious rats, however, showed shortlasting intermittent hypertensive spikes as early as 6 hours after renal artery constriction. In spite of these acute hypertensive peaks, which frequently exceeded 200 mm Hg, intimal lesions, potentially leading to malignant nephrosclerosis, began to appear after only 2 weeks. Thus, the development of nonobliterating, acute focal necroses of intrarenal arteries in the earliest stage of two-kidney, one clip hypertension may be explained by intermittent hypertensive episodes accompanied by a segmental overstretching of the nonadapted vascular bed. On the other hand, the occurrence of obliterating malignant nephrosclerosis in the presence of severe hypertension may also depend on the previously changed composition of the vessel walls.