Sucrose gradient analysis revealed an aldosterone-dependent increase in the incorporation of [(3)H]uridine into a nonmethylated, rapidly labeled RNA that sedimented at 9-12 S. This effect was antagonized by an anti-mineralocorticoid, spironolactone (SC 9420), and was not elicited either by a glucocorticoid, cortisol, or by the inactive isomer, 17-alpha-isoaldosterone. These findings are in accord with the inference that mRNA is induced by aldosterone during the latent period and that the induced mRNA mediates the action on sodium transport.