SummaryPulmonary mechanics were studied with the constant pressure body plethysmograph method in 78 infants during the first year of life. Registrations of breathing frequency, tidal volume, minute volume, dynamic compliance, pulmonary functional resistance, and end-expiratory resistance were made at rest and during carbon dioxide-induced hyperventilation. Data from 70 infants demonstrated strong correlations (P< 0.001) between all pulmonary function parameters and length. Carbon dioxide-induced hyperventilation was achieved mainly by an increased tidal volume. Pulmonary functional resistance did not change with hyperventilation, indicating wider airways during forced ventilation.A special breathing pattern called “subclinical grunting” was found in 16 of 45 infants who were low birth weight but without clinical signs of hyaline membrane disease. The pattern was characterized by a high resistance during most of the prolonged expiration. It was identical to that previously described in cases of hyaline membrane diseases. All re-examined low-birth-weight infants with subclinical grunting developed a normal breathing pattern.