The effect of hydrodynamic stress on intracellular factors of plant cells was investigated. Safflower (Carthamus tinctorius) cells were cultivated in a stirred tank reactor. The aggregate size distribution was determined with an image analyzer and it was confirmed that the aggregate size became smaller in a stirred tank. The resultant changes in the cells were evaluated on the basis of cell number, dry cell weight, cell viability, ATP amount and cell membrane fluidity. The viability was estimated by growth potential, membrane integrity and respiration activity of the cells. The cells were damaged by hydrodynamic stress in the respiration activity before the occurrence of the cell lysis and the damage of membrane integrity. ATP amount decreased as the agitation rate increased. The extent of ATP loss was larger than that of viability loss. These results indicated that hydrodynamic stress affected the metabolic system and that the loss of viability was probably due to this metabolic change. The degree of ATP reduction showed a power law dependence on the power dissipation rate per unit mass, epsilon, which is calculated from agitation rates.