The renal prostaglandins (PGs) are formed mainly in the endoplasmatic reticulum from locally available precursor, arachidonic acid (C20:4). Although the main PG formation occurs in the papilla, significant amounts of PGs are also formed in the cortex. PGs are not stored, but at once released to the cytosol and metabolized by soluble enzymes, 15-OH-PG-dehydrogenase (PGDA), delta13-PG-reductase and PGE-9-keto-reductase. PG metabolism by the PGDH pathway occurs predominantly in cortex. C20:4 can be used to study the biological effects of the renal PG system. C20:4 given to rabbits increases renal biosynthesis of PGs, renal blood flow, predominantly in the juxtamedullary cortex, and plasma renin activity. These effects are inhibited by PG synthesis inhibitors like indomethacin.