The aim of the present study was to determine the effects of dietary arginine supplementation on resistance and responses to ammonia toxicity in common carp (Cyprinus carpio). In the first experiment, the fish were fed with four diets supplemented with 0% (control diet), 0.25% (0.25Arg), 0.5% (0.5Arg) or 1% (1Arg) for 14 days and challenged with 0.7, 0.8, 0.9, 1.1, 1.3 mg/L water unionized ammonia for 3 h. The results showed that 0.7 mg/L ammonia caused no mortality in the treatments, but 1.3 mg/L ammonia killed all the fish. In the second experiment, the fish fed either the control or the 0.5Arg diet for 14 days and exposed to 0.7 mg/L ammonia for 3 h, and plasma ammonia and free amino acids were determined. The results showed that arginine administration significantly increased plasma arginine, urea, ornithine, glutamate, aspartate and histidine, whereas decreased plasma ammonia, citrulline, glutamine and other amino acids except for glycine, asparagine and serine. Ammonia exposure led to significant elevations in plasma ammonia and urea, and significant declines in plasma arginine, ornithine, citrulline and other amino acids. In conclusion, common carp is ureagenic under ammonia exposure and tries to detoxify ammonia by converting to urea, as well as formation of glutamine. To keep glutamate levels constant during ammonia toxicity, common carp may utilize alanine, aspartate, glycine and serine for α-ketoglutarate formation as a substrate for glutamate synthesis. Arginine administration provides high levels of ornithine, which is used to convert ammonia to urea after ammonia toxicity. The benefits of arginine in common carp resistance to ammonia toxicity seem to be related to its stimulatory effects on ureageneis; however, other health benefits of the amino acid (anti-stress, antioxidant and anti-anemic) should not be neglected. © 2019 Elsevier B.V.