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The purpose of this paper was to study the effect of rumen metabolism on the concentration of certain cyclic N-compounds in blood plasma. The plasma from two groups of lambs in the phase of rumen development during early and late weaning was used. As a second model, samples from steers fed at maintenance and after emptying of the rumen via cannula were used. Plasma samples were obtained from the jugular vein in regular intervals. The HPLC-technique was employed as analytical method. The plasma levels of allantoin and of hippuric acid increased about 2.2-3.7 fold and 2.4-6.4 fold in lambs during rumen development. They decreased to about 43% and 13% after emptying the rumen of steers. These metabolites appear to reflect rumen protein synthesis and fibre breakdown and are considered useful estimators of rumen function. Cytidine, uridine, uric acid and hypoxanthine were only influenced in part by rumen metabolism. Plasma pseudouridine was decreased and creatinine was increased in lambs in relation to body mass and remained unaffected in steers.