The effects of experimentally induced rhinovirus and mycoplasmal respiratory tract infections on the pulmonary function of healthy, young, adult volunteers were investigated prospectively. Before inoculation, 12 volunteers were completely normal, whereas 9 had increased frequency dependence of compliance as their only abnormality of pulmonary function. Rhinovirus infection was induced in 8 of the completely normal volunteers, and 5 developed increased frequency dependence of compliance at the time of illness. These changes returned toward normal during the following 2 weeks and were not associated with concomitant changes in any other parameter of pulmonary function, including "closing volume". Three subjects with increased frequency dependence of compliance as their only abnormality before rhinovirus infection developed no significant change in dynamic compliance or any other abnormality in pulmonary function after infection. No changes in pulmonary function were detected in 3 volunteers with experimentally induced mycoplasmal infection. These finding suggest that although rhinovirus infections are associated primarily with upper respiratory illness, they can produce transient peripheral airway abnormalities in previously normal young adults; however, closing volumes, as well as routine pulmonary function studies, may not detect these changes.