We have studied the effect of unilateral nephrectomy alone or in association with experimentally-induced (DOCA-salt treatment) or genetic (SHR) hypertension on the formation of lesions in the contralateral renal artery. These lesions, which form spontaneously with age in small numbers in the renal artery of the control Wistar rat, are characterized principally by an interruption of the internal elastic lamina (IEL) over part or all of the vessel circumference. Such lesions are also present in small numbers in renal arteries of intact WKY and SHR rats. Unilateral nephrectomy increases lesion formation in the remaining renal artery in all groups of rats and this increase is greatest when hypertension accompanies nephrectomy. The increase in lesion formation is accompanied in most cases by a significant increase in length of the remaining renal artery and in all cases by compensatory renal hypertrophy. The possibility that the increased flow in the renal artery irrigating the hypertrophied kidney may be responsible for the increased lesion formation is discussed. Lesions in renal arteries of control and nephrectomized normotensive rats and SHR did not differ greatly morphologically, and lesions containing lipid deposits were observed in all groups. Only in the DOCA-salt hypertension group were severe arterial alterations observed.