Previous work showed that individual airway size, prior to any spasmogen, varied widely in the same animals on different days. The effect of this variable baseline size on the airway response to a subsequent challenge is unknown. The present study examined how the variability in individual airway baseline size in dogs was related to that after methacholine challenge on 4 different days using High resolution Computed Tomography scans. Dogs were anesthetized and ventilated, and on 4 separate days randomly varying between 1 and 8 weeks apart, baseline scans were acquired, followed by a continuous intravenous infusion of methacholine at 3 rates in increasing order (17, 67, and 200µg/min). As the measure of variability, we used the Coefficient of Variation of the four airway luminal measurements of each airway at baseline and at each dose of methacholine. For most airways, there was wide variability both between and within dogs in the response to a given dose of methacholine (CV=33%-38%). Airways with any level of methacholine stimulation had greater variability than at baseline. The airway variability was greatest at the lowest dose of methacholine administered, but was elevated at all the doses. In conclusion, there was substantial day-to-day variability in baseline airway size. Most importantly, the same dose of methacholine to the same individual airway showed even greater variability than at baseline. If we consider that increased heterogeneity may potentiate clinical symptoms, airway response variability may play an important role in the manifestation of airway disease.