Five different volumes of test solutions (1 to 3 ml) containing 15 mg of metaproterenol were placed in each of five different nebulizers. The time to complete nebulization and the amount of drug delivered varied considerably, depending upon the initial volume of solution placed in the nebulizer. Small volumes (1 ml) were almost totally retained in the nebulizer, whereas larger volumes (3 ml) took an unacceptably long time for nebulization. With vigorous agitation, a maximum of 53 to 72 percent of the dose left the nebulizer, but even less (34 to 59 percent) was delivered under simulated clinical conditions. When nebulization was synchronized with breathing, only 0.33 to 0.54 ml of solution (2 to 3.2 mg of metaproterenol) was delivered with 90 deep inhalations. If nebulization was continuous instead of intermittent during the time to take 90 breaths, the majority of the drug was nebulized to the atmosphere.