Persons with hyperreactive airways differ quantitatively from normal persons in their response to bronchial provocation testing with methacholine chloride (Mch). Methacholine chloride has strong hygroscopic properties that necessitate careful dessication, preparation, and handling if accurate results are to be achieved. The rate of hydration of Mch was determined by exposing the drug to various levels of relative humidity. The rate of water uptake was directly related to the availability of water in the atmosphere. At 49% relative humidity, a 0.125%/min gain in weight was noted, which increased to 0.489%/min at 80% relative humidity. Adequate dessication time and technique were determined by noting the time required to return the drug from a hydrated to a dry state. With proper technique, dessication was virtually complete by 4 to 6 h, and 20 to 24 h assured a return to the dry state. From these data, guidelines for preparation and handling of Mch test solutions can be formulated.