TXB2 is released in substantial quantities from aggregating platelets and metabolized during circulation to 11-dehydro TXB2 and 2,3-dinor TXB2.{1410} In rats and rabbits, 2,3-dinor TXB1 has been identified as another urinary metabolite of TXB2.{12600,12599} However in human urine, only trace amounts of 2,3-dinor TXB1 have been identified.{12600} In rats, 2,3-dinor TXB1 is excreted at a much higher rate than 2,3-dinor TXB2 (19.2 ± 4.9 ng/24 hr and 1.6 ± 0.3 ng/24 hr, respectively).{12600} Therefore, urinary 2,3-dinor TXB1 is a suitable marker of thromboxane biosynthesis in rats.