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The present study investigated how the frequency composition of a complex sound differentially influences newborns' and five month-olds' auditory localization. The stimuli were tape recordings of a rattle, filtered to produce 4 stimulus conditions: low frequency band-pass rattle ( less than 1600 Hz ) , mid-frequency band-pass rattle ( 1000-3000 Hz ), high frequency band-pass rattle ( greater than 1800 Hz ), and an unfiltered broad-band rattle. Sound pressure level was varied across trials to eliminate the possibility of any confounding effects as a result of the frequency manipulation. Direction, latency and duration of head turning, alerting and quieting were scored from videotapes of the infant's behavior. Cardiac measures of responding were also taken in order to assess developmental ly the nature of cardiac-somatic relations in infants. In general, five month-olds head turned towards sounds more than newborns, and in contrast to the newborns, they turned their heads exclusively in the direction of the sound source. Significant differences in quantitative aspects of head turning towards a sound emerged, the head turning of newborns being longer in latency and duration. Differences in head turning towards the different frequency sounds occurred in newborns and five month-olds. The pattern of results for newborn head turning towards the sounds revealed an order of correct ' localizability ' proceeding from low ^ medium -4 high/broad frequencies* with no iv significant differences in head turning to the high vs broad-band frequency stimuli. Five month-olds showed an order of localizability proceeding from low/medium high/broad-band frequencies. Crossage comparisons revealed a developmental increase in head turning towards the low frequency stimulus. Five month-olds showed more alerting and quieting to sounds than newborns, and were as likely to alert as to quiet to a sound. In contrast, newborns showed significantly more alerting than quieting. Cardiac measures of responding revealed that heart rate change varied, at each age, as a function of head turning towards a sound. Newborns responded with cardiac decelerations only on those trials in which they did not head turn. On 'head turn' trials they showed no reliable cardiac response. Five month-olds responded with cardiac deceleration on both 'head turn' and 'no head turn' trials. The magnitude of the deceleration, however, was greater on trials in which they did not head turn. Results are discussed with regard to the development of the auditory system, recent findings on developmental changes in sensitivity to frequency information and literature on cardiacsomatic relations.