We have studied the factors of variation of the urinary allantoin excretion with sheep billy goats and goats during digestibility trials. The urinary excretion of nitrogen allantoin (Nal) is highly correlated with the intake of digestible organic matter (DOM) and soluble nitrogen (Ns) for the sheeps billy goats and goats. By multiple regression we have estimated the daily endogenous excretion of Nal at 160 mg (goats) and 60 mg (sheeps and billy goats) respectively 8.5 and 2.6 mg/kg W0.75. The differences observed between sheeps and billy goats (Nal = 27.3 +/- 11.3 and 32.2 +/- 9.4 mg/kg W0.75) agree with the differences observed in the DOM and Ns intakes. The dairy goats eat more than the billy goats (69.5 against 52.0 DOM/kg W0.75 and 412 against 315 mg Ns/kg W0.75) but they excrete less Nal (36.3 mg against 42.3/kg W0.75). During the first two weeks of lactation the goats with negative N balance excrete more Nal than the goats with positive N balance (78 mg Nal against 57 mg/100 g DOM). Although large variations of DOM and Ns, the amount of Nal excreted before and after kidding doesn't vary. In these last conditions the general relation between Nal and intake of DOM and Ns no longer holds. Since the disponible energy (MOD) and soluble nitrogen intake supplies for the rumen microorganisms influence the amount of synthetised microbials protein (and so the amount of nucleic acids entering the duodenum) our regressions confirm the assumption that the excretion of Nal is related with the nucleic acids metabolism. However when Nal is utilised as an index of the nucleic acids catabolism, the factors that are able to create the variations either in the endogenous production of allantoin (uterine involution, bodily mobilisation, foetus growth, mammary gland development) or in the nucleic acids valorization must be taken into account.