Using dogs, cats, and monkeys for test animals and with a concentric cylindrical electrode devised so as to obtain the electric field of any desired area, the author estimated the stimulation threshold as well as calculated the number of Betz cells contained in the electric field by stimulating the cerebral cortex motor area of these animals; 1) Any symptom of motor response could not be observed when dogs, cats and monkeys were stimulated with the concentric cylindrical electrode less than 1mm in diameter. When it was more than 1mm in diameter, some response could be found, but could not get any response sometimes at some spots even if it was more than 1mm in diameter. This caused when the spots had thin density of Betz cells. 2) The stimulation threshold of the forefoot center and the hindleg center in the cortex motor area of dogs is about equal or a little lower in the forefoot center, whereas in the case of cats and monkeys the stimulation threshold of the forefoot center is clearly lower than that of the hindleg center. 3) In order to elicit the motor response it is necessary to stimulate 16-19 Betz cells of the forefoot center and 22-26 Betz cells of the hindleg center in the dog; 18-23 cells of forefoot center and 29-34 cells of the hindleg center in the cat; and 16 cells of the forefoot center and 25 cells of the hindleg center in the monkey. 4) The difference of the stimulation threshold in the same center depends upon the difference in the density of Betz cells in the center. 5) The difference of the stimulation threshold in different centers depends upon the difference in the number of Betz cells required to elicit the motor response.