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We present results of electrical measurements performed both on symmetric and asymmetric membranes in current-clamp conditions. The current—voltage characteristic curve of the membranes shows a reversible conductance transition to a higher level above a critical potential V c. The experimental results are interpreted in the light of the electroporation theory, which allows estimates of the line tension to be made. These estimates are compared to previous experimental findings or theoretical calculations. The behaviour of symmetric membranes of different chain lengths or consisting of mixtures of short and long chains indicates a strong dependence of V c on the chain composition and on the presence of charges on the polar head. The electroporation process is also analyzed in asymmetric bilayers consisting of a charged and an uncharged monolayer, a condition which mimics that of natural membranes …