The reactions of 38 infants to two social stimuli (mother and experimenter (E) smiling and talking) as well as a nonsocial stimulus (a rotating musical mobile) were observed longitudinally when they were 4-, 12-, and 20-weeks old. The durations of four responses were assessed: visual focus, smile, vocalization, and movement. By 12 weeks of age all four responses differentiated among the stimuli but not in the same way. Infants looked at the nonsocial stimulus more than they did at the social stimuli. Each of the other three responses was manifested more to the social stimuli. Infants were more active in the presence of their mothers than they were in the presence of the E, but at 20 weeks they looked at the E more and tended to smile at the E more. These results are discussed in terms of the discrepancy hypothesis.