Around the turn of the last century, there was intensive discussion among physiologists as to whether there was a law that described the phenomena of electrostimulation. In 1892 J.L. Hoorweg became the first to put electrostimulation on a quantitative footing. As early as 1901 G. Weiss formulated a law which made it possible to compare the effect of stimulation pulses of different shape. In 1909 L. Lapicque introduced the terms "rheobase" and "chronaxie", thus giving the "fundamental law of electrostimulation" its final form, which is still valid today. We are now able to give Lapicque's Law a physical interpretation. It is the electric field which is important for initiating electrostimulation by acting directly on the permeability of the membrane. The size of this field is produced by a transformation of the low extracellular field by the membrane. The consequences arising from the theory and our hypothesis are remarkable, and are illustrated for a number of practical problems. Thus, for example, a current of 50 Hz is dangerous if it creates an electric field strength within the heart of 34 V/m or more. For frequencies above 10 kHz the danger threshold increases by 20 dB/octave.