Renally excreted modified RNA catabolites [pseudouridine (ѱ), N2,N2-dimethylguanosine (m22G) and N6-threoninocarbonyladenosine (t6A)] are markers of whole-body rates of degradation of rRNA and tRNA, and are thought to be potential indicators of the resting metabolic rate. To investigate diurnal variations of these RNA catabolites, the amounts of ѱ, m22G and t6A excreted were determined by HPLC of the urine from eight healthy male adults collected over 47-h periods, which were subdivided into the morning (06.00 or 09.00 to 12.00 hours), the afternoon (12.00 to 18.00 hours), the evening (18.00 to 24.00 hours) and the night (00.00 to 06.00 or 08.00 hours), under two different nutritional regimens with 100 or 50 g of protein/day. Furthermore, rates of degradation of rRNA and tRNA were calculated using values for these RNA catabolites. For comparison, the corresponding excretion of creatinine, which originates from the energy metabolism of muscle, and of 3-methylhistidine (m3His), which is an indicator of muscle protein degradation, was determined. Differences in excretion during the collection periods were tested using the Friedmann test. The excretion of ѱ, creatinine and m3His (μmol·h-1·kg-1) altered significantly (P<0.001) during the day. Medians were: for ѱ, 0.21 (morning), 0.19 (afternoon), 0.19 (evening) and 0.18 (night); for creatinine, 8.8, 8.4, 8.0 and 7.3 respectively; for m3His, 0.13, 0.11, 0.12 and 0.10 respectively. The excretion rates of m22G and t6A (nmol·h-1·kg-1) altered, but not significantly, during the day; corresponding medians were: for m22G, 9.0, 8.4, 8.0 and 8.4 respectively; for t6A, 4.3, 4.1, 3.9 and 3.9 respectively. From these results it can be concluded that, in order to assess the average daily rates of degradation of tRNA and rRNA using modified RNA catabolites, urine collection should be carried out quantitatively over 24 h periods. Likewise, the catabolites creatinine and m3His must be determined using 24 h urine samples when average daily excretion values are required.