The authors studied the particle size of aerosols of metaproterenol produced by three different actuators designed for use in patients with endotracheal tubes in place. These were compared with the metaproterenol aerosol produced by the actuator (provided by Boehringer-Ingelheim [BI]) that was supplied by the manufacturer for use in patients whose tracheas are not intubated. The volume of particles in the respiratory size range (1.0–5.1 μm) delivered to the end of the endotracheal tube were measured using adapters designed by Intec (IT), Instrumentation Industries (II), and Monaghan (MAIS). Particle numbers were measured using a CSAS 100 scatering-aerosol laser spectrometer, and volumes were calculated by assuming the particles were spheres. The authors found that the volume of particles in the respiratory range with the IT, II, and MAIS adapters plus endotracheal tube were 11, 31, and 66%, respectively, of the volume produced in the respiratory range by the BI. When particles likely to impact before reaching the lower airways (> 5 μm) were measured, almost none was produced by the adapters plus endotracheal tube, whereas the majority of drug volume in the BI aerosol was in the > 5 μm range. It was concluded that the aerosol produced by different actuators differ from each other, that all three produced less drug in the respiratory range than was produced by the manufacturer-supplied actuator, and that large particles are effectively removed by the adapter plus endotracheal tube.