Inorganic sulfate is a cosubstrate for numerous sulfoconjugation reactions, including sulfation of estrogen steroids in the fetoplacental unit. It is known that the availability of inorganic sulfate can be the rate-limiting factor in these reactions, but fluxes of inorganic sulfate across the maternal-placental barrier have not been well characterized. Therefore, we measured serum inorganic sulfate in matched samples from 46 mothers and fetuses at parturition to identify any maternal-fetal gradient and explore clinical correlations. The concentration of inorganic sulfate, measured by controlled-flow anion chromatography, was significantly higher (p = 0.006) in fetal cord blood [458 +/- 10 microM; mean +/- SE] than in the maternal circulation [431 +/- 19 microM]. That a gradient was not observed for chloride ion rules out sampling artifact as a source of the difference. Maternal and fetal concentrations of inorganic sulfate were highly correlated (r = 0.84, p less than 0.001). No influence was observed for gestational history, newborn weight, sex, or Apgar scores, but values were significantly higher in those with relatively shorter (less than 36 weeks) or longer (greater than 41 weeks) gestations. We demonstrated that a small but significant fetal-to-maternal inorganic sulfate gradient exists at birth, but the origin of this gradient is not known.