The effect of dietary fiber viscosity on apparent ileal nitrogen and amino acid digestibility, proteolytic enzyme activity and digestive organ weights was investigated. Eighteen growing rats were fed for 21 d purified casein-based diets containing carboxymethylcellulose (50 g/kg) of low (20 cP), medium (800 cP) and high (2000 cP) viscosity (LV, MV and HV treatment groups, respectively). Dietary fiber viscosity did not significantly affect apparent ileal (terminal 15 cm of the ileum) nitrogen or amino acid digestibility, trypsin or chymotrypsin activity in the small intestinal contents and pancreatic tissue, aminopeptidase-N activity in the small intestinal contents and tissue, or the weights of the stomach, pancreas, small or large intestines. Intragastric pepsin activity in LV rats was significantly higher than in MV or HV rats (P < 0.01), but fiber viscosity did not affect pepsin activity in the stomach tissue. The intragastric pH of the HV and MV rats was significantly higher than that for the LV rats (P < 0.01). The stomach contents (dry matter) of MV and HV rats were greater than in LV rats (P < 0.05). Delayed passage rate of the more viscous digesta may have resulted in greater absorption of amino acids, because the HV rats had a higher estimated true ileal digestibility than the LV animals for threonine, serine, aspartic acid, glutamic acid, histidine, tyrosine and phenylalanine.