A diffusion cell consists of a donor chan1ber and a receptor chamber between which the skin is positioned (an example of a typ ical design is provided in Figure 1). The cell should provide a good seal around the skin, enable easy sampling and good mixing of the receptor solution in contact with the underside of the skin, and good temperature control of the cell and its contents. Static and flow-through diffusion cells are both acceptable . Normally, donor chambers are left unoccluded during exposure to a finite dose of a test preparation . However, for infin ite applications and certain sce narios for finite doses, the donor chambers may be occluded.