Pre-eclampsia complicates 7 - 10% of pregnancies and constitutes a leading cause of fetal growth retardation and premature birth, as well as infant and maternal morbidity and mortality. The kidney is the primary site of injury resulting in high blood pressure, loss of protein into the urine and decreased kidney function. The release of vasoconstrictors over vasodilators from an abnormal placenta may underlie pre-eclampsia. Nitric Oxide (NO) is an important vasodilator that is thought to play an important role in the kidneys ability to accommodate to a healthy pregnancy. Normal pregnancy in the rat is accompanied by increased production of NO and its second messenger cGMP. There is a parallel increase in renal expression of constitutive nitric oxide synthase (NOS), the enzyme that generates NO from arginine. In the pregnant rat, an infusion of NG-nitro-L-arginine methyl ester (L-NAME), an exogenous inhibitor of NOS, has been shown to replicate some of the hemodynamic features of the syndrome of pre-eclampsia. In a recent animal study, L-arginine supplementation reversed the adverse effects of L-NAME on pregnancy by attenuating the high blood pressure and by significantly decreasing protein loss in the urine. To date, studies of the use of L-arginine supplementation to treat women with pre-eclampsia have been small or uncontrolled and have only assessed blood pressure as a primary outcome measure. We report a single center, randomized, placebo-controlled trial of L-arginine supplementation for the treatment of pre-eclampsia, in which precise physiological techniques have been utilized to assess kidney dysfunction in addition to blood pressure.