Acute bronchoconstriction following isocapnic hyperpnea can be produced in most asthmatics. However, the existence of a late phase response is less certain. We used a canine model of isocapnic hyperpnea to test the hypothesis that this discrepancy is due to differences in the challenge threshold for the responses. Acute phase and late phase bronchoconstriction was measured in 9 dogs after peripheral airway exposure to unconditioned air. Additionally, bronchoalveolar lavage fluid (BALF) was obtained during the late phase response. The acute phase response was a polynomial function with a decreasing slope at higher challenges, whereas the late phase response suggested a minimum threshold of challenge severity was needed to produce late phase bronchoconstriction. BALF leukocyte and eicosanoid concentrations had linear relationships with challenge severity. Our data support the hypothesis that acute and late phase post-hyperpnea responses have different dose-response relationships, a fact that may explain the frequent lack of a late phase response. However, our data suggest that mild inflammation can be induced with relatively lower challenge severity.