The effects of etamsylate on prostaglandin (PG) biosynthesis in microsomes of pregnant human myometrium in vitro have been determined, and compared with those of indomethacin. Both drugs inhibited PG biosynthesis, indomethacin being the more potent inhibitor of the two. Etamsylate inhibited synthesis of 6-oxo-PGF1 alpha, PGF2 alpha, PGE2, and thromboxane B2; increasing the concentration of etamsylate increased the inhibition of synthesis. It is suggested that etamsylate has no anti-cyclo-oxygenase activity, but acts by inhibiting the activity of prostacyclin synthetase, endoperoxide reductase, endoperoxide isomerase, and thromboxane synthetase.