To study the involvement of 6 keto prostaglandin F1 alpha (6 keto PGF1 alpha) and thromboxane B2 (TxB2) in mild pregnancy-induced hypertension (PIH), we measured amniotic fluid and plasma concentration of these prostanoids by radioimmunoassay in PIH subjects and normotensive pregnant controls. The results suggest no difference in plasma or amniotic fluid 6 keto PGF1 alpha or plasma TxB2 in PIH and control subjects. The concentration of TxB2 (mean +/- SE) in amniotic fluid, however, were 189.5 +/- 27.7 for PIH and 107.4 +/- 9.7 for controls (P less than 0.05). This increase in vasoconstrictor TxB2 over vasodilator 6 keto PGF1 alpha may have implications in the hypertensive vascular response of mild PIH.