Simple SummaryThis study aims to investigate the effects of GAA on carcass traits, plasma biochemical parameters, tissue antioxidant capacity, and tissue amino acid contents in finishing pigs. The results of this study indicate that GAA enhanced the plasma biochemical parameters, oxidative status, and amino acid profiles of the heart andlongissimus thoracismuscle in finishing pigs.AbstractThis study aims to investigate the effects of guanidine acetic acid (GAA) on carcass traits, plasma biochemical parameters, tissue antioxidant capacity, and tissue-bound amino acid contents in finishing pigs. Seventy-two 140-day-old (body weight 86.59 ± 1.16 kg) crossbred pigs (Duroc × Landrace × Large White) were randomly assigned into four treatments with six replicate pens and three pigs per pen, which were fed the basal diets supplemented with 0, 0.05%, 0.10%, or 0.15% GAA, respectively. The plasma glucose concentration decreased, and creatine kinase activity and levels of GAA and creatine increased with the dietary GAA concentration. GAA linearly improved creatine content in thelongissimus thoracismuscle (LM) and heart. The activities of superoxide dismutase, total antioxidant capacity, and glutathione peroxidase increased linearly in tissue or/and plasma, while the contents of malondialdehyde and protein carbonyl decreased linearly. GAA improved the contents of multiple-bound amino acids (such as proline or isoleucine) in the myocardium and LM. In conclusion, GAA enhanced the plasma biochemical parameters, oxidative status, and bound amino acid profiles of the heart and LM in finishing pigs.