With another litter the following results were obtained. On the thirteenth day each of the eight members of the litter responded definitely and uniformly to the Galton whistle, set at 5 (probably about 8000 complete vibrations), and to a König steel bar of a vibration rate of 4096 Vs. The largest individuals, for almost always there are noticeable differences in size among the members of a litter, appeared to be most sensitive to sounds. On the fifteenth day and again on the seventeenth unmistakable responses to sound were observed; on the eighteenth the responses were indefinite, and on the nineteenth none were obtained. I continued the tests up to the twenty-eighth day without further indications of hearing. Certain individuals in this litter reacted so vigorously to the loud sound produced by striking the steel bar a sharp blow and also to the Galton whistle, during a period of five days, that I have no hesitation in saying that they evidently heard during that period of their lives. Other members of the litter seemed to be less sensitive; their reactions were sometimes so indefinite as to leave the experimenter in doubt about the presence of hearing. A third litter, which developed very slowly because of lack of sufficient food, first showed unmistakable reactions to sound on the twenty-first day. On this day only two of the five individuals reacted. The reactions were much more obvious on the twenty-second day, but thereafter they became indefinite. Still another litter, which consisted of one female and four males, began to exhibit the quick, jerky movements, already mentioned, on the fourteenth day. On the morning of the fifteenth day three members of the litter definitely reacted to the tone of the steel bar, and also to the hammer blow when the bar was held tightly in the hand of the experimenter. My observations were verified by another experimenter. Two individuals which appeared to be very sensitive were selected for special tests. Their reactions were obvious on the sixteenth, seventeenth, and eighteenth days; on the nineteenth day they were indefinite, and on the twentieth none could be detected. Some individuals of this litter certainly had the ability to hear for at least five days. A sixth litter of four females and two males first gave indications of the change in behavior which by this time I had come to interpret as a sign of the approach of the period of auditory sensitiveness, on the seventeenth day. I had tested them almost every day previous to this time without obtaining evidence of hearing. The tests with the steel bar and the Galton whistle were continued each day until the end of the fourth week without positive results. To all appearances the individuals of this litter were unable to hear at any time during the first month of life.