Apparatus and method whereby ultra low frequency dielectric measurements allow a determination to be made of the parameters (dielectric constant, thickness and conductivity) characterizing electrically distinct layers with membranes such as bimolecular lipid or protein membranes (i.e. the major components of the membranes of living cells).The location and effect of chemicals such as anaesthetics, tranquilizers, hormones and other substances of pharmacological and general biological interest which are adsorbed into these or other membranes can be determined. The effects of these chemicals can be quantitatively specified in terms of this effect of the substructural dielectric parameters.