Urinary GH and somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) excretion were measured in 12-h urine collections obtained from 43 infants (27 stable preterm infants and 16 healthy fullterm infants) and 31 normal children, aged 3-17 yr. Urinary Sm-C/IGF-I was excreted as the free hormone, since no binding of radiolabeled Sm-C/IGF-I to any urine protein with a mol wt similar to those described for plasma Sm-C/IGF-I-binding proteins was found. The preterm infants excreted significantly more urinary GH [13.5 +/- 2.1 (+/- SE) ng/kg.12 h] than either the fullterm infants (5.3 +/- 1.6 ng/kg.12h) or the children (0.27 +/- 0.02 ng/kg.12 h; P less than 0.01). The mean urinary Sm-C/IGF-I excretion in the preterm infants (98.9 +/- 7.5 mU/kg.12 h) was comparable to that in fullterm infants (87.6 +/- 9.7 mU/kg.12 h); both groups excreted significantly more urinary Sm-C/IGF-I than children (28.4 +/- 2.1 mU/kg.12 h; P less than 0.01). The group differences were similar when the results were expressed in terms of creatinine excretion. Urinary GH excretion correlated positively with urinary Sm-C/IGF-I excretion (r = 0.68). The higher output of these peptides in rapidly growing infants and their positive correlation in urine provide additional support for the Sm hypothesis.