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