This study was designed to investigate if animals exposed to urban levels of air pollution develop pulmonary hyperresponsiveness and to test if this change was reversed after moving the animals to a nonpolluted environment. One hundred twenty male Wistar rats were kept in (a) São Paulo (polluted environment) for 3 months (SP3); (b) Atibaia (clean region), for 3 months (A3); (c) São Paulo for 3 months and then Atibaia for a further 3 months (SPA6); (d) Atibaia for 6 months (A6). After the exposure period, the rats were submitted to dose-response curves to inhaled methacholine. Older animals (SPA6 and A6) had lower responses to methacholine in terms of respiratory system resistance when compared to the animals studied after 3 months of experiment (SP3 and A3). However, the response in terms of respiratory system elastance of the SP3 group was significantly (P = 0.0004) greater than those of the other three groups. Our results suggest that the environmental conditions of the large urban centers can induce pulmonary hyperresponsiveness in rats that can be reversed when the animals are removed to a nonpolluted area.