An approximate equivalent circuit is presented which describes the electrical characteristics of a metal electrode coated with a weakly conducting material and placed in an electrolyte solution. The model is compared with experimental results obtained over the frequency range 10-2 to 106 Hz. For the tests, copper electrodes were coated in PVC and immersed in KCl solutions. Experiments on this model system showed that over several decades of frequency the total circuit admittance was dominated by the properties of the coating material, which in turn were sensitive to the electrolyte molarity. For example, the results displayed a loss process attributable to the absorption of solute ions by the film. The AC admittance technique also appears to be sensitive enough to allow an investigation of very thin Langmuir-Blodgett (LB) films. Sputtered copper electrodes coated in three monolayers of barium stearate and immersed in KCl solution are shown to have a dielectric constant of about 2.7. This value compares favourably with previously quoted values for 'dry' films, suggesting that overall film quality is good. Such measurements can be used to determine the stability of LB films in aqueous media; an important consideration for film deposition and for some sensor applications.

The homeostatic regulation of bilayer order is a property of functional importance. Arguably, it is best studied in those organisms which experience and must overcome disturbances in bilayer order which may be imposed by variations in temperature of hydrostatic pressure. This article reviews our recent work on the adaptations of order in brain membranes of those fish which acclimate to seasonal changes in temperature or which have evolved in extreme thermal or abyssal habitats. The effects of temperature and pressure upon hydrocarbon order and phase state are reviewed to indicate the magnitude of the disturbances experienced by animals in their environments over the seasonal or evolutionary timescale. Acclimation of fish to altered temperature leads to a partial correction of order, while comparison of fish from extreme cold environments with those from temperate or tropical waters reveals a more complete adaptation. Fish from the deep sea also display adaptations of bilayer order which largely overcome the ordering effects of pressure.

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The effects of high pressure on the phase state and molecular structure of pure lipid bilayers are discussed. The relations of delta H, delta S and delta V in phase transitions are straightforward and are discernible in heterogeneous bilayers in natural membranes. The effects of pressure on the dynamic properties of bilayer constituents are less clearly understood, but order parameters obtained at pressure by different techniques show agreement. The extent and significance of hydration is poorly understood. Four physiological functions are discussed: passive permeability, active transport, membrane excitability and synaptic transmission. It is shown that a full interpretation of the kinetic effects of pressure on these processes requires much more detailed molecular information than is available at present.