Discrete changes in the human newborn's sucking behavior in response to acoustic stimulation was studied. In order to obtain a sensitive comparison measure for stimulus conditions, control trials were randomly interspersed with stimulus trials. To prevent temporal biasing effects, variable interstimulus intervals were used, and stimuli were presented contingent on the infant's sucking state on each trial. Results from 30 Ss clearly demonstrate the presence of two dichotomous responses: (1) response initiation to the stimulus in the presence of non-sucking; and (2) response cessation to the stimulus in the presence of active-sucking. No differences were found among three groups exposed to qualitatively different stimuli of the same intensity, and there was no evidence of habituation of either response. © 1968.