This study tested the hypothesis that intrathoracic bronchi intermittently constrict during coughing and attempted to elucidate the effect on explosive flow. The subjects were 21 dogs having undergone tracheostomy. In the first group A (n = 7), the diameter of the fifth-generation bronchus was measured with a balloon-tipped catheter and the change during coughing was analyzed. In the other group (n = 14), the dogs were vagotomized and coughing was simulated by sequential application of positive and negative airway pressures (sham cough). The effects of the bronchial constriction, elicited by the stimulation of vagus efferent fibers, on explosive flow and airway pressure of sham cough were analyzed. The bronchus was constricted in explosive phase of spontaneous coughing in all the dogs of the first group. When cough bouts were repetitively developed, bronchial constriction and phrenic burst developed simultaneously. The intermittent bronchial constrictions fused and virtually acted as tonic constriction. In the second group of dogs the explosive flow of sham cough consisted of two phases; a short bout followed by a near-constant flow. When the bronchus was constricted, the explosive flow was still biphasic in 12 dogs and an exponential decay pattern formed in 2 of them. In these 12 dogs, the peak explosive flow slightly but significantly decreased (mean ± SD, 1.39 ± 0.23 vs. 1.34 ± 0.23 l/s) and airway pressure in the segmental bronchus became smaller (−1.18 ± 0.53 vs. −0.15 ± 0.94 cmH₂O). We concluded that intermittent bronchial constrictions act as tonic one during coughing. Bronchial constriction slightly decreased the peak explosive flow and moved the choke point to the proximal airway.