The effect of dietary peptides on gut endogenous nitrogen (N) flow (ENFL) and amino acid (AA) flow (EAAFL) was studied. Semisynthetic diets containing enzyme-hydrolyzed casein (HC; 11%) or a free AA mixture devoid of Asp and Ser (A1) or Gly and Ala (A2) were formulated to have similar AA compositions except for the excluded AA and similar dietary electrolyte balances (Na(+)+K(+)-Cl(-)). A protein-free diet (PF) served as a control. Sprague-Dawley rats were given the diets 8 times/d for 10 min each hour for 7 d. Rats were killed and digesta were sampled (6 observations within each group) along the intestinal tract 6 h after the first meal on d 7. EAAFL and ENFL, estimated with reference to the dietary marker TiO(2), were determined directly (PF, A1, and A2) or after centrifugation and ultrafiltration of the digesta (HC). Endogenous flows of Asp and Ser or Gly and Ala did not differ (P > 0.05) in any of the intestinal sections between rats fed PF and A1 or PF and A2, respectively, except in the stomach where Ser flow was greater for rats fed A1. Ileal endogenous flows for most of the AA and for N were higher (P < 0.05) for rats fed the HC diet compared with those for rats fed the PF, A1, or A2 diets, except for Phe, Tyr, Lys, which did not differ among the groups. Ileal EAAFL and ENFL were not influenced by body N balance per se but were affected by the presence in the gut of dietary peptides derived from casein.