The polyamines regulate RNA levels by stimulating the hydrolytic activity of a number of bacterial and mammalian ribonucleases (RNases). These effects cannot be duplicated by metal ion. Polyamines can also control RNA levels by a mechanism involving RNase and purine inhibitors of this enzyme. The attachment of a 3 prime terminal polyadenylic acid $ (poly(A)) segment to an RNA molecule serves to inhibit the RNase mediated degradation of the non-polyadenylated portion of the molecule. Inhibition of RNase activity, and, therefore, substrate degradation, is dependent upon the length of the poly(A) segment. Spermidine reverses that poly(A) induced inhibition of RNase activity and thus, in the presence of this polyamine, there is an increase in the hydrolysis of the non-polyadenylated portion of the RNA. Spermidine can control, therefore, the rate at which a messenger RNA-like molecule is degraded by any of several RNases. Polyamines also serve to protect RNases from thermal inactivation and to renature inactivated enzyme.