Response to the Inhaled bronchodilator, metaproterenol, was evaluated In 28 outpatient infants and young children with cystic fibrosis (CF)(mean age, 16 months) and In 22 normal control children (mean age, 13 months). Lung function was assessed from partial expiratory flow volume curves generated by the rapid compression technique and was quantitated by the maximal expiratory flow at functional residual capacity cYmaxFRC). For the normal control group there was no significant change In VmaxFRC after the aerosol of either normal saline or metaproterenol. At baseline, the group of Infants with CF had significantly lower values of VmaxFRCthan did the normal control Infants (202 versus 273 mils, p< 0.05). The CF group demonstrated no significant change from baseline VmaxFRC after the aerosol of normal saline. However, after metaproterenol the CF group had a significant Increase (p< 0.001) In VmaxFRC, which eliminated the difference In VrnaxFRC between the CF and normal control groups (267 versus 276 mils). We conclude that Infants and young children with CF have Increased bronchomotor tone and that bronchoconstrlctlon represents a significant component of the airway obstruction present In patients with CF at this age.