The efficacy of a new valved spacer device, the Babyhaler inhaler (Glaxo) for administering metered dose inhaler treatment via a facemask to infants was assessed. In a double blind, single dose study, salbutamol (800 micrograms) or placebo were given on separate days to 12 sedated, sleeping, wheezy infants during a symptom free interval. Lung function was measured before and after administration and the bronchial response to aerosol challenge with methacholine was then assessed using the squeeze technique. A small increase in heart rate and a drop in arterial oxygen tension followed salbutamol administration. No other change in lung volume or air flow obstruction was detected. Bronchial responsiveness decreased significantly after the administration of salbutamol by Babyhaler, the PC30 (provoking concentration of methacholine causing a 30% fall in maximal flow at functional residual capacity by the squeeze technique) increasing from a median of 3.8 g/l after placebo to 12.5 g/l after salbutamol. The Babyhaler is an effective device for administering bronchodilator to wheezy infants. The small scale of the response may be attributable to the uncertain effect of beta agonists in this population. Furthermore, pulmonary deposition of inhaled aerosols may be reduced in nose breathing, sleeping infants.