Intrapulmonary distribution of a peptice-phospholipid (KJ,4) surfactant administered through an adapter permitting maintenance of positive end-expiratory pressure was compared with distribution by instillation with disconnection from mechanical ventilation in 10 surfactant-deficientMacaca mullatapreterm infants. Animals received KL4surfactant (200 mg/kg) when the arterial to alveolar (oxygen ratio) (a/Ao2) was ≤0.22 (approximately 50 min after birth) on mechanical ventilation. Six rhesus infants received bolus instillation of two half doses of KL4surfactant through an cndotrachcal tube adapter over 10–15 s while maintaining positive end-expiratory pressure (0.4 kPa) accompanied by turning to the right and left lateral positions for 60 s. In four rhesus premature infants KL4surfactant was injected as two half-dose boluses through the endotracheal tube with disconnection from mechanical ventilation while positioning the infant rhesus monkey in the right and left lateral positions for 30 s of mechanical ventilation between instillation. Acute effects on oxygen saturation were monitored, and physiologic measures of a/Ao2, mean airway pressure, and the ventilatory efficiency index were monitored over the 12-h study. Intrapulmonary distribution of KL4surfactant was determined using dye-labeled microspheres or [3H]dipalmitoyiphosphatidylcholinc-labeled KL4surfactant, measured by colorimetry or by scintillation counting. Lungs of cach monkey were processed into 50 ± 5 pieces to determine distribution of radiolabel or microspheres and for scanning electron microscopy. The drop in oxygen saturation was greater among monkey infants discoanected from the ventilator for surfactant instillation. Surfactant distribution in lung pieces inside a distribution interval 0.6–1.4 of the mean was greater among infants having instillation accompanied by positive end-expiratory pressure (p< 0.04). whereas the proportion of lung pieces receiving ≤ 10% of the normalized mean number of microspheres/mg of lung tissue was significantly greater in the group removed from the ventilator for surfactant instillation. The a/Ao2, lung compliance, and mean airway pressures improved in both treatment groups; however, the ventilatory efficiency index increased more above pretreatment values among infants treated using the endotracheal adapter with positive end-expiratory pressure (p< 0.04). More homogeneous distribution of KL4surfactant results when this surfactant is administered while maintaining positive end-expiratory pressures during surfactant instillation.