Airway responsiveness to inhaled methacholine was assessed before and after bronchial challenge with ultrasonically nebulized hyperosmolar saline (UNHS), and these changes were correlated with the development of late asthmatic responses (LAR). Sixteen subjects with mild to moderate asthma had two consecutive methacholine challenges before and one after a cumulative-dose challenge with UNHS. Twelve of these subjects also had a single-dose hyperosmolar challenge to document the occurrence of LAR and determine if UNHS had a significant cumulative-dose effect. If a LAR was observed, a control day without challenge completed the study. Responsiveness to methacholine was similar on the 2 baseline methacholine challenges with a provocative concentration producing a 20% fall in forced expiratory volume in one second (PC20) (mean +/- SEM) of 1.11 +/- 0.94 and 1.16 +/- 0.94 mg.ml.1 (r: 0.98). However, it was significantly increased after the inhalation of UNHS with a PC20 (mean +/- SEM) of 0.57 +/- 1.00 mg.ml.1 (p less than 0.001). Two subjects developed a late fall in forced expiratory volume in one second (FEV1) of 19 and 46% after hyperosmolar challenge. In this last subject, the LAR, not reproduced on the control day, was associated with a marked post-UNHS change in PC20, going from a baseline of 4.4 to 0.7 mg.ml.1 after UNHS. The % fall in FEV1 following the dose-response hyperosmolar challenge and the single-dose hyperosmolar challenge were not different, with mean values +/- SEM of 34.9 +/- 2.2 and 35.8 +/- 4.1, respectively, (p greater than 0.5). In conclusion, airway responsiveness to methacholine may increase following hyperosmolar saline inhalation, often unrelated to LAR.