The present study was aimed at investigating whether there are changes in the expression of nitric oxide synthase (NOS) in relation with the unclipping-induced fall of blood pressure in two-kidney, one clip (2K1C) hypertension. Male Sprague-Dawley rats were made 2K1C hypertensive by clipping the left renal artery for four weeks. Sham-clipped rats served as control. The expression of endothelial constitutive (ec) NOS proteins and tissue levels of NO metabolites were determined in the kidney. Systolic blood pressure was significantly increased in clipped rats compared with that in the control. The development of hypertension was associated with decreases in the expression of ecNOS proteins and tissue levels of NO metabolites in the clipped kidney. The blood pressure at twenty-four hours after removal of the renal arterial clip fell to the control level. Accordingly, in the unclipped kidney, the expression of ecNOS proteins and tissue contents of NO metabolites were increased to the control level. The contralateral kidney was not affected by the development or reversal of hypertension. It is suggested that an enhanced expression of ecNOS in the unclipped kidney is an important component in the reversal of renovascular hypertension.