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="">