The nucleic acids digested by ruminants are essentially of microbial origin. Absorbed purines are extensively degraded and excreted in urine as allantoin, uric acid, xanthine and hypoxanthine. The measurement of the urinary purine derivatives could be used to estimate the uptake of purines and hence that of microbial protein. 1) Automated methods for measurements of purine derivatives were improved. A HPLC method for determining total adenine and guanine was used to measure the purine contents of mixed rumen bacteria and the digestibility of microbial purines in sheep. 2) Endogenous excretions of purine derivatives by sheep and cattle of different physiological states were measured using animals nourished by intragastric infusions of VFA and casein. The species difference in and the effects of changes in protein supply on the endogenous excretion were studied. 3) A microbial nucleic acid extract was infused into the abomasum of lambs at 6 levels. The subsequent urinary excretion of purine derivatives was examined. The results suggested that exogenous purines were utilised by the sheep. A model is proposed to describe the relationship between purine derivative excretion and purine uptake. 4) Allantoin was infused intravenously to sheep and the changes in plasma concentration, nephric tubular re-absorption and urinary excretion of allantoin were studied. Results showed that plasma allantoin was rapidly removed and a constant proportion of the allantoin entering the blood was excreted in urine of sheep. 5) Secretion of allantoin and uric acid into the gut via saliva was quantified in sheep. The possible decomposition of the allantoin in the rumen by microbes in the rumen fluid and in the rumen wall of sheep was examined <i>in vitro</i> and <i>in vivo</i>. Allantoin infused into the rumen or abomasum of sheep and cattle was not recovered in urine. 6) A model for calculation of microbial protein available to sheep is proposed. It is suggested that a different model should be used for cattle. These models were applied in two feeding experiments with sheep and steers.