The current theories of the membrane electrical properties of the giant algal cells of the Characeae are reviewed. The experimental techniques for the determination of such properties are discussed and a comparison between these techniques, for both the D.C. and A.C. case, are made. In the latter case it was necessary to obtain the time dependent solution of the A.C. cable theory and the numerical behaviour of this solution. From this comparison it is proposed that symmetric external current injection together with the 0.42L technique is a very suitable method for these studies. Using this method the membrane electric parameters of Nitella translucens were determined. The effects of external pH, 4-dinitrophenol and A.C. of different frequencies on these parameters were studied. The effects of external pH and the presence of DNP on the vacuolar pH were also determined. It was found that the experimental observations favoured the Spanswick theory for the Characean membrane electric properties, but modified to account for the membrane resistance being pH independent. The observed effects of DNP favoured the mechanism for the action of DNP proposed by Duncan and Croghan. The observed A.C. frequency dependence of the membrane resistance and experiments on the punch-through effect are inconsistent with the Coster double fixed charge model of membrane structure.