Four Göttinger miniature pigs and five domestic pigs (Landrace) of similar body weight fitted with an ileocaecal re-entrant cannula were given each a cereal-based diets with or without fish meal (Treatments 1 and 2). The diets contained increasing levels of partially hydrolysed straw meal and pectin (2:1 w/w). On day 3, 5 and 7 after the last administration of ¹⁵NH₄Cl given with the diets during five days the ileal flow of endogenous nitrogen (N) was measured using the atom %¹⁵N excess in urinary N as indicator for that in endogenous N. The ¹⁵N-enrichments in urinary N, in trichloroacetic acid (TCA)-soluble N of blood plasma as well as in TCA-soluble and -preciptable N in the pancreas and small intestine were nearly the same. Furthermore, the contribution of endogenous N to total, TCA-preciptable and bacterial N of ileal digesta was not affected by the three collection periods. Urinary N seems to be the easiest accessible and valid indicator for determination of the endogenous N under these experimental conditions. Neither the fibre supplements nor the N intake affected the daily ileal flow of endogenous N. Contrary to total bacterial N , the ileal flow of bacterial N of endogenous origin was not affected by the level of protein intake. The contribution of endogenous N associated with bacteria to total endogenous N ranged between 0.43 to 0.56 (Treatment 1) and 0.40 to 0.53 (Treatment 2). On the other side, the proportion of endogenous N in bacterial N increased from 0.48 to 0.57 (treatment 1) up to 0.56 to 0.79 (Treatment 2). It is concluded that endogenous N represents an easily available N source for bacterial protein synthesis at both levels of N intake.