We studied rupture of planar lipid membranes. We applied short electric field pulses across planar lipid membranes. One pulse charges the membrane and give rise to electric forces. Above a critical threshold voltage rupture of the membrane is induced and a fast discharge of the membrane across the defect is observed. An analysis of the time course of the voltage gives information on the energy barrier of the membrane against rupture and allows conclusion on the kinetics of the defect widening. Our set-up allows to follow the kinetics from the ns to s range. We observed that in planar lipid membranes held under tension the defects spread very fast with about 0.1–0.3 m/s. On the other hand, creating a polymer network by polymerizing actin onto the lipid membrane cause a drastic slowing down of the rupture process by more than 3 orders of magnitude. Under such extreme conditions the rupture process is determined by the membrane viscosity.