We determined the effect of stress on the structure, number and morphology of cells in the head kidney with those of control fish (unstressed). In unstressed animals, the head kidneyconsisted ofnormal and regularly distributed lymphatic tissue, hematopoietic tissue, blood vessels, melano-macrophages, and interregnalmelano-macrophage enlargement of the interrenal cellular and nuclear areas and nuclear diameter in 24 h after exposure to the handling stress, there was a decrease in the number of melano-macrophages and in thesize of the interrenal cellular and nuclear areas and nuclear diameter,We identified six typeserythrocytes, lymphocytes, monocytes,neutrophilic granulocytes, basophilic granulocytes, and thrombocytes in the head kidney of control fish. The handling stress led to an erythrocytes. Interestingly, the number of erythrocytes, neutrophilic granulocytes, and basophilic granulocytes decreased continuously after the stress, whereas the number of lymphocytes and monocytes increased 0.1 h after the stress but then decreased, while the number of thrombocytes increased at all times after the stress. Our results suggest that stress significantly affects the structure and cell morphology in the head kidney. We also noted the return of normal biological function within 24 h for several of the indices.