The use of a physiologically conducive receptor fluid is preferred although others may also be used provided that they are justified . T he precise composition of the receptor fluid should be provided. Adequate solubility of the test chemical in the receptor fluid should be demonstrated so that it does not act as a barrier to absorption. In addition, the receptor fluid should not affect skin preparation integri ty. In a flow-through system, the rate of flow must not hinder diffusion of a test substance into the receptor flu id. In a static cell system, the fluid should be continuously stirred and sampled regularly. If metabolism is bei ng studied, the receptor fluid must support skin viability throughout the experiment.