The in vitro synthesis of prostaglandins E2 and F2 alpha by renal cortex, medulla and papilla was measured in normal rats and in rats receiving either a low or a high sodium intake for 14 days. The production of both prostaglandins was unchanged in the cortex. In the medulla, both low and high sodium intakes led to a similar decrease in prostaglandin E2 synthesis in vitro, but prostaglandin F2 alpha synthesis was unchanged. In the papilla, a low sodium intake increased prostaglandin E2 synthesis. The activity of prostaglandin E2 9-ketoreductase, a cytosolic enzyme catalysing the conversion of prostaglandin E2 to prostaglandin F2 alpha, was unchanged in cortical preparations. In medullary slices, prostaglandin E2 9-ketoreductase activity was decreased by both sodium depletion and loading. In the papilla, prostaglandin E2 9-ketoreductase activity was slightly decreased by sodium loading and increased with sodium depletion. These results obtained in the rat are at variance with findings in the rabbit. The role played by prostaglandin E2 9-ketoreductase in the regulation of prostaglandin biosynthesis during changes of sodium balance remains controversial.