Analogous to childhood-onset asthma in humans, rats may develop a chronic asthmalike phenotype, depending on their genetic background and the age at which they experience a viral airway injury. Brown Norway rats develop a postbronchiolitis asthmalike phenotype that may be prevented with supplements of interferon-gamma (IFN-gamma); we hypothesized that the normally resistant F344 rat strain would develop the asthmalike phenotype if the IFN-gamma response were suppressed during viral illness. Weanling F344 rats were pretreated with anti-IFN-gamma or control antibody, and inoculated with Sendai virus or vehicle. Anti-IFN-gamma treatment reduced lung IFN-gamma and increased IL-4 mRNA during the infection. Physiologic studies performed 8 wk later revealed premature airway closure (p = 0.03) and elevated specific pulmonary resistance (p