During bronchoprovocation testing with methacholine, induced changes in airway geometry are known to affect sites of drug deposition. However, it is not known if changes in these sites determine measured responsiveness. We assessed the importance of sites of deposition as determinants of reactivity by comparing particle behavior in two subject groups with and without hyperresponsiveness. By administering radiolabeled aerosols of similar aerodynamic characteristics to methacholine aerosol, we measured the deposition pattern in terms of the specific central to peripheral ratio (sC/P) before and after methacholine inhalation (sC/P1 and sC/P2, respectively) and thereby quantified the changes in deposition sites that occur during the course of a typical bronchoprovocation test. Subjects whose FEV1 decreased by 20 percent or greater were classified as methacholine responsive (MR; nine subjects), and the remainder were classified as non-methacholine responsive (NMR; seven subjects). The two groups had similar baseline FEV1 percent predicted (FEV1 percent) and initial deposition patterns (sC/P1) with particles depositing primarily in peripheral airways (mean +/- SE; sC/P1 1.43 +/- 0.070 and 1.39 +/- 0.65, MR and NMR, respectively, p = NS). Following methacholine inhalation, the deposition pattern changes markedly for all subjects with particles depositing primarily in central airways (sC/P2 2.58 +/- 0.24, p = 0.001, and 2.15 +/- 0.22, p = 0.001 from baseline, p = NS between groups) By definition, the MR subjects had a significantly greater change in FEV1 than the NMR subjects. Preferential deposition in central airways occurs in all subjects during bronchoprovocation testing and does not significantly determine methacholine responsiveness.