The mechanism of action of the auditory signal in increasing responsiveness to a visual-auditory stimulus in chicks of 14-48 hr was examined. Stimulus effectiveness of an auditory signal dissociated from the visual stimulus was less than proximate auditory signal presentation but greater than a silent visual stimulus. Familiarity with the auditory signal increased and decreased responding under proximate and dissociated presentation, respectively. These data were interpreted as confirming the influence of auditory stimulation via increased arousal, and as indicating the possibility of selective responding to specific features of the auditory stimulus when its localization at the visual stimulus is possible.