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 Vc. 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 Vc 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. Therefore it is possible to analyze the electrical forces acting on the uncharged monolayer due to the presence of charges on the other one, under several ionic-strength conditions. It is shown that the instability arises in the uncharged monolayer, while the coupling between the two monolayers triggers the electroporation process.