The levels of prostaglandins (PG S) E<sub>2</sub> and F<sub>2</sub>α in different parts of the rabbit kidney were determined to observe the effect of sodium and indomethacin. After the pretreatment with injections of saline or with indomethacin, tissues from inner and outer medulla and cortex were separated, extracted, analyzed for PGE<sub>2</sub> and PGF<sub>2</sub>α by radioimmunoassay. In the normal rabbit kidney, the greatest amount of PG was found in the inner medulla. Saline injections appeared to increase PGE<sub>2</sub> (but not PGF<sub>2</sub>α), especially in the inner medulla. Repeated injections of saline, on the other hand, markedly reduced PGE<sub>2</sub> in the inner medulla but increased outer medullary PGE<sub>2</sub>. Indomethacin reduced the production of PG S in all kidney segments. These results suggest the bidirectional effect of sodium on PG concentration in the rabbit kidney. Acute administration of sodium may directly stimulate the synthesis of PGE<sub>2</sub> in the inner medulla but chronic stimulation with sodium may alter the pattern of PGE<sub>2</sub> synthesis.