To evaluate the effect of saline instillation prior to tracheal suction on lung mechanics in mechanically ventilated newborn infants, we studied pulmonary mechanics in nine infants with respiratory distress syndrome (RDS) and nine infants with meconium-aspiration syndrome (MAS) at a mean postnatal age of 3 days. Pulmonary mechanics were measured at 10 minutes prior to, and at 10, 20, and 30 minutes after tracheal suction with saline instillation. Suction and study protocol were repeated within 12 hours without saline instillation. The sequence of the study with and without saline instillation was randomly assigned. In infants with RDS, tracheal suction had no effect on pulmonary compliance or airway resistance with and without saline instillation. In infants with MAS, there was no change in compliance after tracheal suction with and without saline instillation. Airway resistance decreased by 35% after tracheal suction with saline instillation in infants with MAS; tracheal suction without saline instillation had no effect on airway resistance. We conclude that saline instillation into trachea as commonly done during tracheal suction has no deleterious effects on lung mechanics in newborn infants.