The increased TXA2 and decreased PGI2 seen in the toxemic pregnancies suggest the imbalance of arachidonic metabolism. Thromboxane (TX) B2 was thought to be a suitable parameter of TXA2 production in vivo. However, recently it has come to be recognized that a large amount of TXB2 is readily produced and decomposed to 11-dehydro TXB2 during blood sampling. In this study, a new RIA method was established, capable of measuring the urinary 11-dehydro TXB2 level. (1) The samples were measured with a 125I-labeled RIA kit for 11-dehydro TXB2 (Amersham, U.K.). In this assay system a highly specific antibody to 11-dehydro TXB2 was used. (2) The values for urinary 11-dehydro TXB2 measured by the RIA correlated closely with those measured by gas chromatography-mass spectrometry (r = 0.95). (3) The mean 11-dehydro TXB2 value in non pregnant was 2.62 +/- 1.71ng/mg Cr, The mean values in the first, second and third trimesters of pregnancy were 4.57 +/- 2.45ng/mg Cr, 5.75 +/- 2.81ng/mg Cr, 6.33 +/- 2.60ng/mg Cr, respectively. (4) The mean 11-dehydro TXB2 value in toxemic pregnancy was 4.13 +/- 1.86ng/mg Cr, which was significantly lower than that in normal pregnancies. (5) There was a significant negative correlation (p less than 0.0001) between the urinary 11-dehydro TXB2 concentration and the blood pressure.