Prostaglandins (PG) were synthesized at similar rates by bovine mesenteric arteries and veins; viz., ca. 200 ng/g wet weight after one hour of incubation. After synthesis, PGE and PGF compounds were released from slices of arteries and veins into the incubating medium; PG were not detected in the walls of these blood vessels. Arachidonic acid, the precursor to PGE-2 and PGF-2-alpha, did not affect PG synthesis, whereas meclofenamate, an aspirin-like agent, decreased synthesis in arteries and veins by 90%. The PG biosynthetic capacity of these blood vessels is high, as indicated by greater than 20% conversion of (1-14C)-arachidonic acid to radiolabeled PG. Under control conditions in both arteries and veins, synthesis of PGE-2 exceeded that of PGF-2-alpha twofold. Bradykinin selectively increased the synthesis of a PGE-like substance in arteries and of a PGE-like substance in veins.