Cell shrinking occurs if human erythrocytes are suspended in hypertonic (greater than 0·17 m) solution of sodium chloride. A general simulation of this process has been performed using the technique of finite element analysis and values of some parameters that describe the normal human red cell. Fick's law of diffusion and the law of conservation of mass have been used to calculate the changes with time of the cell volume and the consequent change in solute concentrations in the cell and the unstirred water layer around the cell. The simulation readily permits changing the variables (such as the diffusion coefficient) or the boundary conditions (such as the temperature and the tonicity of the medium).