A comparison of aerosol delivery has been made between two open-vent Pari jet nebulizers. Intermittent and continuous delivery were compared for one of the nebulizers. Ten healthy volunteers inhaled 99mTc-labelled diethylenetriamine penta-acetic acid (DTPA) aerosols on three occasions. The Pari LC device was operated both intermittently (by a manual interrupter that generated aerosol only in synchrony with inhalation) and continuously. The Pari LL nebulizer was operated only in the intermittent mode. A system of inspiratory and expiratory valves was fitted to each nebulizer in order to direct airflow. Both nebulizers were powered by Pari Boy compressors. The mean (SD) whole lung deposition for the LL nebulizer was 11.1 (4.0)% of the nominal dose, compared to 15.3 (12.8)% and 12.8 (7.9)% for the LC used with intermittent and continuous operation, respectively. These differences were not statistically significant. Regional deposition patterns within the lungs were similar for the three nebulizer systems. These data show that efficient nebulizer systems using relatively low power compressors are possible, and suggest that continuously operated open-vent nebulizers may be designed to give lung deposition comparable to that achieved by nebulizers fitted with manual interrupters.