The effects of viral infections of the upper respiratory tract on airway dynamics were studied in twelve normal subjects. Initial studies were performed on the third to fifth day of a clinically apparent viral infection of the upper respiratory tract. Expiratory flow rates, airway resistance (Raw) and static pressure-volume measurements were obtained. Dynamic lung compliance (Cdyn) was measured with standard methods at breathing frequencies of 0.3, 0.6, 0.9 and 1.2 cps. Measurements were repeated four to eight weeks after recovery from the infection. Flow rates and airway resistance (Raw) were normal in all subjects and did not change significantly on recovery. In eight Cdyn was normal during and after infection. Four others, in whom lung compliance was normal during infection, had frequency dependent compliance four to eight weeks later. This returned to normal after an additional six weeks. The occurrence of frequency dependent compliance in the presence of normal Raw is evidence of small airway involvement. We conclude that viral infections of the upper respiratory tract involve the lower airways of the lung in some normal subjects and produce changes in airway dynamics which are measurable for several weeks.