Hydrostatic-pressure- and transmembrane-potential-induced deformations of glyceryl monooleate (GMO) and bovine brain phosphatidylserine (PS) bilayer lipid membranes (BLMs) were investigated by simultaneous optical and electrical measurements. The investigation of a large number of samples established subtle variations in the observable parameters (appearance of concentric optical interference fringes, capacitance, voltage-dependent capacitance, resistivity, and voltage-dependent resistivity changes as a function of hydrostatic pressure and applied potential) of separately formed, but otherwise identical BLMs