To investigate physiologic alterations in respiratory function associated with chest physiotherapy, arterial blood gases, respiratory patterns, lung mechanics, and functional residual capacity were measured in 13 neonates (weights 1.25 to 3.20 kg) during the control period, after vibration of the chest and suctioning, after hyperventilation, and two hours after suctioning. Compared to control values, mean PO2decreased significantly after suctioning to 43 mm Hg and increased significantly after hyperventilation to 78 mm Hg. There was a significant decrease in inspiratory resistance and a trend toward decrease in expiratory resistance after suctioning, with return to control levels after hyperventilation. Respiratory rate increased significantly after suctioning. Functional residual capacity, dynamic lung compliance, and tidal volume, as well as PCO2 and base excess, were not changed appreciably throughout the protocol. Because of potentially severe hypoxemia, this study suggests that suctioning and hyperventilation are not warranted on a routine basis in infants recovering from respiratory diseases.