Abstract: Using high performance liquid chromatographic method, for the first time, this study determined the concentrations of17 serum amino acids of theYangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis) inTian-e-zhou Baiji NatureRe-serve and the Baiji Dolphinarium of Institute of Hydrobiology, Chinese Academy of Sciences.The results showed that the meanconcentrations of 14 serum amino acids (Aspartic acid Asp, Glutamic acid Glu, Serine Ser, ArginineArg, Glycine Gly, Threo-nineThr, Alanine Ala, Isoleucine Ile, Leucine Leu, Phenylalanine Phe, Valine Val, Lysine Lys, Tyrosine Tyr and CysteineCys) of captive porpoises were significantly higher (p<0.0001 or 0.05) than those of free-ranging animals, except for ProlinePro, MethionineMet and Histidine His.No significant differences of serum amino acid concentrations were noted between gen-ders and between mature and immature free ranging Yangtze finless porpoises.Glu of captive Yangtze finless porpoise was thehighest, then was the Asp and Lys within the 17 serum amino acids.Glu of free-ranging Yangtze finless porpoise was the highesttoo, then was the Lys and Asp.Met was the lowest in both captive and free-ranging Yangtze finless porpoise.The ratio betweenessential amino acid (EAA) and nonessential amino acids (NEAA) was 0.83 and 0.92 in free-ranging and captiveYangtze fin-less porpoises, respectively.There was significant difference between the ratios (p<0.01).