ABSTRACT In a number of studies on the reduction potentials of living tissues and cells, the method of immersing the material under investigation in solutions of oxidation-reduction indicators has found considerable employment. Under controlled conditions and with sufficient knowledge concerning the physico-chemical characteristics of the indicator systems, this method is of value for determining the magnitude of extracellular reduction potentials. Its use, however, for the determination of intracellular reduction potentials has been hampered by the lack, among other things, of knowledge concerning the penetrating power of the indicators into living cells. Results obtained from immersion experiments alone have proved to be inconsistent. An example of this is given by J. and D. Needham (1) who examined a series of indicators and showed that cells become coloured when immersed in solutions of indicators which should be reduced intracellularly, while they give no sign of coloration in solutions of other indicators which should remain oxidised. These authors recognised the discrepancy but did not examine it more thoroughly.