When cultures of Escherichia coli growing in minimal medium (M-9 salts, 0.5% glycerol, 10 mug/ml of uracil) are shifted to minimal medium supplemented with 50 mug/ml of l-methionine, they exhibit an abrupt increase in their relative rate of synthesis of ribonucleic acid (RNA). The rate of growth of the cells is not increased by such a shift. Sucrose density gradient analyses of cell extract after the shift showed that the effect was exerted equally on all classes of RNA. The initial relative rate of methylation of RNA, as judged by incorporation of radioactivity from l-methionine-methyl-C(14) into RNA, is high in cells recovering from chloramphenicol inhibition, intermediate in cells grown in minimal medium, and low in cells grown in medium containing methionine. In cells recovering from chloramphenicol inhibition, methionine decreases the lag and increases the rate of conversion of ribonucleoprotein particles to mature 70S ribosomes. Turnover of RNA under these conditions, as judged by loss of C(14)-labeled methyl groups, is minimal. These results lead to the inference that methionine increases the relative rate of RNA synthesis by increasing the rate of conversion of ribonucleoprotein particles to ribosomes and thereby decreasing the availability of ribonucleoprotein particles for regulating RNA synthesis by a feedback mechanism.