Leuciscus waleckii is a cyprinid in the subfamily Leuciscinae and is distributed widely in rivers and lakes ofNortheast, Northwest, and Northern China. A unique feature of this species is its resistance to high alkalinity, whichallows it to survive and adapt to the extreme environment of Lake Dali, Inner Mongolia, with carbonate alkalinity >53.57 mmol/L (pH 9.6). Previous studies have explored the mechanisms of the extreme alkaline tolerance usingphysiological, chemical, population genetics, and transcriptomics methods. We assessed the effects of acclimating L.waleckii to different environmental alkalinity levels and exposure times on plasma free amino acids levels. L. waleckiijuveniles were acclimated to alkalinities of 18 mmol/L, 30 mmol/L, and 50 mmol/L for 22 d and 60 d, and plasma levelsof 15 free amino acids were examined and compared between the different alkalinity groups and the control. Theresults showed that total plasma free amino acid contents increased with alkalinity and exposure time, and that the levelsof nonessential amino acids changed the most. Among the nonessential amino acids, proline and valine contentsincreased significantly with increasing alkalinity (P<0.05); proline increased 23 times more than the control in the50 mmol/L alkalinity group. These results suggest that amino acids play an important role as an energy source duringacclimation of L. waleckii to high alkalinity conditions.