We studied 10 normal volunteers with single-breath nitrogen tests before and after methacholrne-induced bronchoconstriction. On the first day we challenged each subject with increasing methacholine concentrations to obtain a 20% drop in the partial forced expired volume in 1 s/partial forced vital capacity ratio. This resulted in an increased residual volume (RV) from 1,536 ± 138 to 2,371 ± 216 ml (mean ± SEM, p < 0.01). On the following day we did single-breath nitrogen tests from RV, RV + AV (AY is the individual’s RV increase after bronchoprovocation) and after a similar methacholine rechallenge. Baseline slope of phase III was 0.88 ± 0.11% N2/l (mean ± SEM), 1.36 ± 0.22% N2/l with the RV + AY test (p < 0.01), and 2.84 ± 0.68% N2/l with bronchoconstriction (p < 0.05). The results show that the changes in RV per se accounted for 24% of the increase in the slope of phase III seen with bronchopro