A diffusion cell for use in determining the absorption rate of a study substance through a permeable membrane. The cell includes a receptor chamber which contains receptor fluid to receive study substance absorbed through the membrane. The receptor chamber includes a fluid inlet and a fluid outlet. A rotary stirrer is situated adjacent the fluid outlet. The stirrer is rotated so as to rotate the receptor fluid in the receptor chamber in such manner that the fluid pressure is greater at the outlet than at the inlet, so that the stirrer defines a pump which pumps receptor fluid outwardly through the outlet, then through a detector, and then back into the receptor chamber through the inlet. The detector determines the concentration of study substance within the receptor fluid.