Rabbit neonates were tracheotomized on day 27--30 of gestation, and lung mechanics during spontaneous ventilation was registered with a fluid-filled esophagus catheter for pressure recording and a pneumotachograph connected to the tracheal cannula via a Fleisch tube. The first breath of all neonates had a very long inspiratory phase (less than or equal to 1.12 sec), with insufflation pressures amounting to 42 cm H2O in the mature animals. The expiration phase of the first breath was usually 'passive', i.e. there was no positive expiratory esophagus pressure. In animals delivered on day 27--28 there was, as a rule, no difference between inspiratory and expiratory tidal volume during the first few breaths, whereas such difference was regularly registered in mature animals; this phenomenon probably corresponds to the establishment of FRC. Dynamic lung compliance increased with both gestational and postnatal age, up to 60 min after delivery. Shortly after birth, pulmonary resistance was higher in immature animals than in fullterm ones, but at later intervals, there was no difference between survivors of various gestational age.