Using a noninvasive passive flow volume technique to measure respiratory system resistance (Rrs) and compliance (Crs), the effects of two inhaled bronchodilators, salbutamol (beta 2-agonist) and ipratropium bromide (muscarinic blocker), were studied in 17 premature infants ranging in age from 19 to 103 days. All were ventilator dependent with chronic lung disease. The pretreatment Rrs was high (range 0.15 to 0.27 cm H2O X ml-1 X sec). After administration of salbutamol, the mean group Rrs decreased 24% +/- 11% (mean +/- SD), and Crs increased significantly, 17% +/- 21%. The response to ipratropium bromide in five infants did not differ significantly from their responses to salbutamol. Nebulized saline solution had no bronchodilator effect in five infants. These results suggest that in infants with early and late chronic lung disease, bronchospasm can be partially alleviated by inhaled bronchodilators.