Inspiratory resistance (R_INSP) and reactance (X_INSP) were measured for 7 min at 5 Hz in 10 subjects with mild asymptomatic asthma and 9 healthy subjects to assess the effects of airway smooth muscle (ASM) activation by methacholine (MCh) and unloading by chest wall strapping (CWS) on the variability of lung function and the effects of deep inspiration (DI). Subjects were studied at control conditions, after MCh, with CWS, and after MCh with CWS. In all experimental conditions X_INSP was significantly more negative in subjects with asthma than in healthy subjects, suggesting greater inhomogeneity in the former. However, the variability in both R_INSP and X_INSP was increased by either ASM activation or CWS, without significant difference between groups. DI significantly reversed MCh-induced changes in R_INSP both in subjects with asthma and healthy subjects, but X_INSP in the former only. This effect was impaired by CWS more in subjects with asthma than in healthy subjects. The velocity of R_INSP and X_INSP recovery after DI was faster in subjects with asthma than healthy subjects. In conclusion, these results support the opinion that the short-term variability in respiratory impedance is related to ASM tone or operating length, rather than to the disease. Nevertheless, ASM in individuals with asthma differs from that in healthy individuals in an increased velocity of shortening and a reduced sensitivity to mechanical stress when strain is reduced.