Many suggestions have been made regarding protective methods against lightning, without any quantitative tests of safety. As a fundamental study on lightning protection, the lethal threshold energy for the artificial lightning impulse was investigated, using 14 rabbits. The voltage and current impulse had a 1 microsecond wave front and 40 microsecond wave tail. The imposed energy was measured by simultaneous photographic recording of both voltage and current waves on separate oscilloscopes. The impulse voltage was applied to head, neck, upper chest, and lower chest while the right hind limb was grounded. Artificial lightning current impulses were fed through the skin of the animals by making the surface flash over so that the surface flash was monitored carefully to delete the data of this case which gives an erroneous energy value from the statistical analysis. The lethal threshold energy was found to be 157.6 +/- 54.1 Joule (J), and the percentage of survival was 25% when 160 J impulse energy was given. Impulses of 160 J were then applied to the heads of 21 other rabbits maintained on an artificial respirator. Under these conditions the percentage of survival increased significantly to 47.6%.