Output, droplet size (by laser instrument), and nebulization time have been compared in vitro for eight individual ultrasonic nebulizers (DeVilbiss Pulmosonic) and eight individual jet nebulizers (DeVilbiss 646), the latter operated by compressed air at flows of 6 and 12 L/min. A solution of hypertonic (7 percent) saline was nebulized. The ultrasonic nebulizer retained a higher "dead" volume of solution on completion of nebulization (p less than 0.05), but the increase in saline concentration was less marked than for the jet (p less than 0.01). The mass of NaCl released as aerosol was similar for the ultrasonic and for the jet at 6 L/min but was increased for the jet at 12 L/min (p less than 0.05). There was a fivefold interindividual variation in output for the ultrasonic. Droplet mass median diameters for the ultrasonic (mean 5.4 micron) were slightly lower than those for the jet at 6 L/min (mean 6.0 micron, p less than 0.05) but were higher than those for the jet at 12 L/min (mean 3.7 micron, p less than 0.01). The ultrasonic emitted virtually no droplets less than 2 micron diameter and may be unsuitable for applications requiring high yields of fine particles for delivery to the peripheral lung regions.