The objective of this study was to determine if reduced-protein amino acid (AA) supplemented diets alter growth and morphology of intestinal mucosa in growing pigs. Twenty-four Yorkshire-Landrace growing pigs (37.0 kg) were assigned to one of 4 diets in a randomized block design: 15% crude protein (15CP), 12CP+synthetic amino acid (SAA), 9CP+SAA and 6CP+SAA. Levels of SAA added were calculated to meet true digestible AA requirements for the growing pig. Diets were offered twice daily for 24 days. Animals were weighed at beginning, on days 7, 14 and 21. Pigs were euthanized two h post-meal and mucosal samples from duodenum, jejunum and ileum sections were collected. During weeks 1 and 2, reducing dietary CP concentration from 15 to 6% resulted in a decrease in ADG and G/F (P < 0.05). During week 3, only pigs fed the 6%+SAA diet had lower ADG and G/F than that of pigs fed other dietary treatments (P < 0.05). There were no differences among dietary treatments in duodenal RNA, DNA and protein concentration. In the jejunum and ileum, mucosa concentrations of protein and protein/RNA ratio were lower in pigs fed reduced protein diets (P < 0.10) compared to that of the Control diet. The reduction of dietary CP concentration from 15 to 6% tended to decrease (P < 0.08) and decreased (P < 0.05) the crypt depth and villus width in the duodenum and jejunum. Villus height and villus surface area in duodenum, jejunum and ileum were not affect by reduction of dietary CP concentration. In the duodenum, free alanine and glycine increased with reduction in dietary CP (P<0.05). Reducing CP increased free lysine, methionine and threonine concentration (P<0.05). For peptidebound (PB) AA, only lysine and threonine increased with reduction in CP. In jejunum, reducing CP led to lower peptide-bound histidine, isoleucine, leucine, phenylalanine and valine concentration (P<0.10). In the ileum, free asparagine, serine, tyrosine, arginine, histidine, leucine, phenylalanine and isoleucine decreased with reducing CP (P<0.05). For PBAA, glycine, serine, isoleucine, leucine, lysine, phenylalanine and valine tended to decrease with reduction of dietary CP (P<0.10). In conclusion, reduced-CP+SAAsupplemented diets lead to biochemical and morphological modifications of intestinal mucosa. These modifications may be important physiological adaptations to allow pigs fed reduced-CP+SAA diets to sustain a normal whole-body growth, but not when CP levels falls 5% below the recommended CP levels.