Two experiments were conducted to determine the effects of dietary NDF on apparent ileal and fecal digestibility and portal flux of nitrogen (N) and amino acids, and on N retention in growing pigs. In four equal portions (at 0600, 1200, 1800, and 2400) barrows on Treatment B received a basal diet, based on casein, cornstarch, and dextrose, at a feeding level of 2.6 times energy for maintenance. Barrows on Treatment B+NDF received an additional amount of 15% (wt/wt) of purified wheat bran NDF (pNDF). In Exp. 1, four ileally cannulated barrows (40 to 75 kg) were used in a crossover arrangement comprising two treatments and three periods. The addition of pNDF decreased ileal N digestibility from 94.1 to 88.9% (P < .001), whereas ileal digestibility of most amino acids was 2 to 5.5 percentage units lower (P < .001). Utilization of ileally digested N increased from 64 to 72% with the addition of pNDF, presumably because of the contribution of pNDF to the energy supply. In Exp. 2, three barrows (30 to 54 kg) fitted with catheters in the portal vein and the mesenteric vein and artery were used in a crossover arrangement comprising two treatments and five periods. Portal absorption of nutrients was derived by multiplying the porto-arterial plasma concentration differences by portal vein plasma flow. The pNDF did not significantly affect the absorption of ileally digested amino acids and the portal flux of ammonia and urea. The results showed that addition of NDF reduced amino acid digestibility, but not the portal flux of digested amino acids, and NDF energy presumably improved utilization of ileally digested amino acids.