The effect of dietary fiber viscosity on the excretion of endogenous nitrogen and amino acids from the small intestinal lumen of growing rats was investigated. Rats were fed for 12 d protein-free diets containing 5% cellulose (negligible viscosity) or 5% carboxymethylcellulose of low (20 cP), medium (800 cP) or high (2000 cP) viscosity, as the sole dietary fiber source. As dietary fiber viscosity increased from 0 to 2000 cP, there was a significant (P < 0.05) linear increase in the flow of endogenous nitrogen, aspartic acid, serine, glutamic acid, proline, threonine, glycine, alanine, isoleucine, histidine, valine, methionine, leucine and lysine at the terminal ileum. The amino acid composition of the ileal digesta was unaffected by changes in fiber viscosity. It seems that either endogenous protein of a similar origin was secreted in larger amounts or that the digestion and absorption of endogenous amino acids was inhibited as dietary fiber viscosity increased. There was a significant (P < 0.05) linear increase in the concentration of sialic acids relative to chromic oxide in the small intestinal contents with increasing fiber viscosity, indicating an increase in mucoproteins.