Prostaglandin biosynthesis from eicosa-8, 11, 14-trienoic acid in microsomes from bovine seminal vesicles is inhibited by relatively high concentrations of hydroxyl radical scavengers: dimethyl sulfoxide, n- and t-butanol, and methional. Methional is a more effective scavenger than t-butanol and dimethyl sulfoxide, two compounds which are more miscible with water than methional. The synthesis of both PGE and PGF is inhibited with incubation systems that promote PGE formation and with incubation systems that promote PGF formation. Furthermore, dimethyl sulfoxide and methional inhibit arachidonic acid-induced platelet aggregation, a reaction involving endoperoxide biosynthesis. The water soluble alcohol, ethanol, stimulates PGF biosynthesis when it is added in the same concentration range as t-butanol. Thus hydroxyl radical scavengers inhibit biosynthesis when their effective concentrations are high and stimulate biosynthesis when their effective concentrations are low. The results of this study and other studies where low concentrations of hydroxyl radical scavengers stimulate both prostaglandin biosynthesis and lipid peroxidation are consistent with a mechanism involving the hydroxyl radical both in the generation of singlet oxygen and the elimination of hydrogen peroxide.