Using double-labeled isotope techniques, it can be shown that aldosterone induces the synthesis of several proteins in the mitochondria-rich (MR) cells of the toad's urinary bladder. Induced proteins have been identified both in the plasma membrane (mol wt = 170,000, 85,000 and 12,000) and the cytosol (mol wt = 36,000, 12,000 and 6,000) fractions of these mucosal cells. We have also shown that aldosterone (Aldo) induces the synthesis of a class of RNA having the properties of messenger RNA (mRNA). mRNA isolated from Aldo-treated mucosal cells was used as template in a cell-free protein-synthesis system prepared from rabbit reticulocytes. Preparations charged with mRNA from Aldo-treated cells synthesized two proteins that were not labeled when mRNA from control tissues was used as template. The electrophoretic mobility of one of these proteins was similar to an Aldo-induced membrane protein (mol wt = 70,000) found in the intact tissue.