The unstabe bioregulator thromboxane A2 (TXA2) is generated from the prostaglandin (PG) endoperoxide PGH2 by an enzyme called thromboxane synthetase. Thromboxane synthetase is present in many tissues including blood platelets, lung, spleen and brain. The formation of TXA2 and the C-17 hydroxy fatty acid HHT from PGH2 has been investigated, with emphasis on kinetics and inhibitors. It has been shown that HHT (12-Hydroxy-5,8,11-heptadecenoic acid) is not a breakdown product of TXA2 but that TXA2 breaks down exclusively into thromboxane B2. Our results also show that thromboxane synthetase is not an isomerase but rather a bimolecular reactions is involved in the formation of TXA2 and perhaps HHT.