Inorganic sulfate (SO4) is an essential metabolite for the synthesis of sulfated mucopolysaccharides and steroid sulfates in the fetus and placenta. The authors' previous study of pregnant women at delivery revealed a substantial increase in serum SO4compared with nonpregnant adults. To determine whether or not this difference was related to altered renal handling, creatinine clearance and SO4reabsorption was measured in 43 women in the third trimester of pregnancy and in 22 nonpregnant control subjects. Serum SO4was 32% higher in the pregnant women than in control subjects. Urinary excretion of SO4was unchanged, but absolute reabsorption of SO4was significantly higher. Blood pressure, serum creatinine, and serum chloride were significant predictors of serum SO4. As a group, women with hypertension (diastolic blood pressure greater than 80 mmHg) had a significantly higher serum SO4than those with normal blood pressure. One result of the increased SO4reabsorption by maternal kidney is that it generates a larger maternal reservoir on which the fetoplacental unit can draw for its biosynthetic needs.