Urinary excretion of the non-reusable modified RNA catabolites pseudouridine (psi), 7-methylguanine (m7Gua) and N2,N2-dimethylguanosine (m2(2)Guo) was measured in preterm infants and in adults. The values (in mumol/mmol of creatinine) were: for preterm and 'small for gestational age' infants (n = 26; number of samples = 38) psi = 164 (SD 32), m7Gua = 39.1 (SD 9.0), m2(2)Guo = 10.6 (SD 2.1); for adults (n = 32) psi = 25.3 (SD 3.1), m7Gua = 4.8 (SD 0.89), m2(2)Guo = 1.53 (SD 0.42). Our measurements were compared with an expectation derived from the average cellular distribution of psi, m7Gua and m2(2)Guo between rRNA, tRNA and mRNA. m2(2)Guo occurs exclusively in tRNA, psi in both rRNA and tRNA, and m7Gua in all three RNA classes, in proportions which can be estimated for the steady state. Urinary excretion of psi and m2(2)Guo should reflect their steady-state distribution, since rRNA and tRNA have been shown to have similar turnover rates in mammalian tissues. We conclude that we can use the excretion of m2(2)Guo to assess whole-body tRNA turnover. Since tRNA contains psi in a constant proportion to m2(2)Guo, the proportion of urinary psi stemming from tRNA can be estimated, and the remainder (approximately 60-65%) is an indicator of rRNA turnover. Finally, the excretion of m7Gua far exceeds the proportion predicted to come from rRNA and tRNA. We ascribe this excess (approximately 60-70% of the total) to the turnover of the mRNA 'cap'-structure, which is typical for all higher organisms. mRNA turnover is known to be much higher than that or rRNA or tRNA.(ABSTRACT TRUNCATED AT 250 WORDS)