Whole-body degradation rates of transfer, ribosomal, and messenger RNA were determined noninvasively in 3-, 6-, 10-, 14-, and 18-y-old female and male subjects (n= 14 per age group per sex) under normal living conditions. The method for determining the RNA degradation rates is based on measuring the renal excretion rates of special RNA catabolites (modified ribonucleosides and nucleobases) by HPLC. Resting metabolic rates were calculated for the same subjects by their body weights using formulas taken from literature. We found high correlations between the degradation rates of the different RNA classes (micromoles per day per kilogram body weight) and the resting metabolic rate (kilojoules per day per kilogram body weight): in females (n= 70),r= 0.75–0.82 and in males (n= 70),r= 0.68–0.79 (p< 0.0001). We conclude that a causal relationship exists between the whole-body degradation rates of the different RNA classes and the resting metabolic rate. Therefore, in healthy subjects noninvasive determinations of RNA degradation rates could be very useful to assess the resting metabolic rate.