We compared the response to isocapnic hyperventilation (ISH), where both cooling and drying of the mucosa occur, with the response to inhaling aerosols of hypertonic saline (HS), where airway osmolarity increases without airway cooling. We studied nine subjects on two days. For ISH, subjects ventilated at 70% of their estimated maximum voluntary ventilation (MVV). For HS, they inhaled aerosols of 2.7, 3.6, or 4.5% saline. The concentration that was used depended on the rate of ventilation during ISH. For both challenges the stimulus was given for one minute. Forced expiratory volume in one second (FEV1) was measured once between each minute, and the challenge ceased when the FEV1 did not change for two successive minutes. A plateau in FEV1 occurred after 8.1 +/- 2.4 (mean +/- 1 SD) min of ISH, and 8.3 +/- 2.4 min of HS. The lowest FEV1 (% predicted) after ISH was 45 +/- 16% and after HS was 51 +/- 18% (r = 0.93). However, the maximum responses occurred after the final challenge and were not the same as the plateau. For HS, the plateau represented 89 +/- 11% of the maximum response which developed within one minute of the final challenge. For ISH, the plateau was only 56 +/- 26% of the maximum response, which developed within 5.2 +/- 2.9 min after challenge. The similarities in the response to these challenges are consistent with the hypothesis that ISH induces asthma via hyperosmolarity. The delayed response to ISH suggests that cooling may delay the response to hyperosmolarity.