Total respiratory resistance (RT) was measured before and after nebulised alpha-adrenergic stimulant therapy in 8 children aged 4 to 18 months who had the clinical symptoms of acute viral croup. In 7 children there was a mean fall in RT of 30% after treatment, associated with an improvement in their clinical condition. This improvement was shortlived, the resistance returning to pretreatment levels within 30 minutes. The remaining child showed no improvement after phenylephrine but was subsequently found to have acute epiglottitis. Nebulised water did not produce any change, indicating that the response was not due to moisture alone.