Measurement of urinary orotidine and orotic acid after an oral allopurinol challenge is an important diagnostic test for ornithine carbamoyltransferase deficiency that is sometimes used in infants (< 1 year of age), although there is little information on normal test results in this age group. We found higher orotidine excretion in normal infants than in older children given the test, whereas orotate excretion was similar in both groups. The increased orotidine excretion appears to be due to the use in the infants of higher allopurinol doses per kilogram of body weight than in the children. The normalized-dose dependency of the orotidine response extends even to adult age. Thus, dose-normalized responses should be used in the test and there is no need for careful age-matching of the controls.