Summary Spectrophotometric determination of RNA and isotopic determination of the incorporation of uridine 5′-monophosphate H3 into RNA have shown that aldosterone stimulates RNA-synthesis in vivo. This occurs during the time aldosterone exerts its antinatruretic action. Actinomycin D, an inhibitor of RNA-synthesis, blocked both of these effects. These observations are consistent with the hypothesis that the antinatruretic action of aldosterone is mediated via DNA directed synthesis of messenger-RNA which in turn stimulates the synthesis of a protein involved in the transport of sodium.