The PD entering the circulation blood can also be from the degradation of tissue nucleic acids. We call this fraction “endogenous PD” measurement of the magnitude of endogenous excretion has been made with the acid of the technique of intragastric infusion, or the technique of replcenment of digesta entering the small intestine. The endogenous excretion of PD is three tims higher in cattle than in sheep per kg of metabolic weight (150 and 530 umol/kg w.075 per day for sheep and cattle respectively). Cross species comparison is also interesting. Sheep, goats, pigs and humans are very similar in the magnitude of endogenous excretion expressed on basis of metabolic weight. The difference in tissue distribution of xanthine oxidase may be the reason for these species differences. Cattle have high xanthine oxidase activities in most tissues including the blood, whereas sheep have low xanthin oxidase activities in most tissues and none in the blood. The high xanthine oxidase activity will divert more of the purines released from the tissue nucleic acid degradation away from the salvage pathway into the degradation pathway.