Previous studies have shown that the presence of nonspecific bronchial hyperreactivity (NSBHR) in transplanted subjects is associated with the development of bronchiolitis obliterans, which suggests that NSBHR in these subjects may involve the peripheral airways. We investigated this question by studying the effects of methacholine on the distribution of ventilation using single-breath washouts in 15 heart-lung transplant recipients; 17 nontransplanted subjects with NSBHR were studied for comparison. All subjects had normal baseline lung function, and seven transplanted subjects displayed NSBHR. Methacholine induced a similar decline in FEV1 and specific airway conductance in the two groups. In contrast, whereas methacholine produced similar increases in the slope of the alveolar plateau for SF6 (SSF6) and He (SHe) in the nontransplanted subjects, it always produced greater increases in SHe than SSF6 in the transplanted subjects. This suggests that in the latter, methacholine-induced bronchoconstriction made the distribution of ventilation more heterogeneous in peripheral airways. This involvement of small airways may help in understanding why assessing bronchial reactivity in transplanted subjects provides information on the presence of a pathologic process affecting the bronchioles, and hence on the risk of progression to bronchiolitis obliterans.