The current study aimed to investigate the effect of sur-plus dietary arginine on polyamine and amino acid metab-olism and accretion of proteins and lipids, as previous studies have demonstrated a lipid-reducing effect following surplus arginine supplementation in viscera as well as increased muscle growth. Four plant protein-based diets were given increasing concentrations of L-arginine, from 21.1 to 36.1 g kg -1 DM, and fed to quadruplicate tanks of adult Atlantic salmon (1.1 kg) for 12 weeks. No effects on growth or deposition of fat or protein were observed. Free amino acid concentrations and related metabolites were unaffected in the liver, except for urea, while concentra-tions in muscle and plasma reflected production of argi-nine metabolites. Polyamine concentrations were unaffected in liver, muscle and white adipose tissue (WAT), as were the abundance and activity of spermi-dine/spermine N1-acetyltransferase (SSAT), the rate-limit-ing enzyme in polyamine turnover. Gene expression demonstrated differential regulation of ornithine decarbox-ylase (ODC) in liver and WAT, although overall little effects were observed on gene expression. Liver S-adeno-sylmethionine (SAM) concentrations decreased with argi-nine supplementation. We suggest that adult Atlantic salmon have adapted to surplus arginine, and the main long-term effect appears to be increased concentrations of arginine metabolites.