The reactions of babies to aircraft noise were studied by means of electroplethysmography (PLG) and electroencephalography (EEG). It was found that the babies whose mothers had moved to the area around the Osaka International Airport before conception (group I) or during the first five months of pregnancy (group II) showed little or no reaction on PLG and on EEG to aircraft noise. In contrast, babies whose mothers had moved closed to the airport during the second half of the pregnancy or after birth (group III or IV) and the babies whose mothers lived in a quiet living area (group V) reacted to the same auditory stimuli. The babies in groups I and II showed differential responses depending on whether the auditory stimuli were aircraft noise or music. Abnormal PLG and EEG were observed in the majority of babies living in an area where noise levels were over 95 dBA. This suggests that the deep sleep of the babies living in such an area was disturbed.