Decreased free water excretion and the development of interstitial edema are recognized characteristics of preeclampsia. However, the pathophysiology of decreased urine excretion in preeclampsia is presently controversial: diminished glomerular filtration, renal arteriolar spasm, elevated plasma vasopressin levels, and plasma volume contraction have been suggested as etiologies. We studied seven pregnant patients with a diagnosis of mild preeclampsia to assess the role of vasopressin, serum protein, and glomerular function in the renal excretion of water. The ability to excrete a water load was significantly and directly correlated with serum albumin (P < 0.05) and protein (P < 0.02) concentrations. Neither plasma vasopressin nor creatinine clearance correlated with water excretion. The similarity of preeclampsia and the nephrotic syndrome with regard to the renal excretion of water is discussed.