Effects of dopamine injection on the hemocyte count,phenoloxidase activity,serine proteinase activity,proteinase inhibitor activity and α2-macroglobulin-like activity in L.vannamei were studied.Results showed that dopamine injection resulted in a significant effect on the parameters measured (P＜0.05),while no significant difference was observed in the control group (0.85％ NaCl).In the experimental groups,the hemocyte count reached the minimum in 3 h; granular and semi-granular cells became stable after 12 h and hyaline cells and the total hemocyte count became stable after 18 h.Phenoloxidase activity reached the minimum in 6 h,and then became stable after 9h.Serine protease activity and proteinase inhibitor activity reached the minimum in 3 h,and α2-macroglobulin-like activity reached the maximum in 3 h,and all the three parameters became stable after 12 h.The results suggest that the activating mechanisms of the proPO system triggered by dopamine are different from those triggered by invasive agents or spontaneously activated under a normal physical condition.