To assess non-specific bronchial reactivity the effect of inhaling ultrasonically nebulised solutions of distilled water and hypotonic (0.3%), isotonic (0.9%), and hypertonic (2.7%, 3.6%) saline was investigated in 10 asthmatic patients and nine normal subjects. Expired ventilation and the maximum percentage fall in forced expiratory volume in one second (FEV1) were recorded. The sensitivity to the inhaled solutions was determined by measuring the ventilation required to induce a fall in FEV1 of 20% from the prechallenge value. Hypotonic and hypertonic but not isotonic solutions caused a significant fall in FEV1 in the asthmatic subjects. Normal subjects showed no response to either distilled water or 3.6% saline, the only solutions with which they were challenged. The method used for this challenge is rapid, simple, and inexpensive and provides a new means of diagnosing non-immunologically mediated bronchial hyperreactivity.