The location of large and small nerve cells taking part in the realization of the protective reflex in the snail subesophageal nerve ganglion circle was studied by the method modified of the retrograde transport of cobalt ions. The data were compared with those of the previous electrophysiological studies of the protective reflex mechanism. A conclusion is made that this response is realized by a system consisting of 9 large and 60-80 small nerve cells.