The total and regional deposition of aerosol has been compared using four nebulizers; the Pari Boy 37.80, Pari IS-2, RespirGard II and Penta-Sonic. The aerosol was radiolabelled with [99Tcm]DTPA, administered to ten healthy subjects and the distributions monitored by gamma scintigraphy. Median lung aerosol depositions expressed as percentages of the doses initially loaded into the nebulizers were: Pari IS-2 19%, Pari Boy 37.80 13%, RespirGard II 9% and Penta-Sonic 2%. The ratios of the peripheral to central lung deposition, however, were greater with the RespirGard II and Penta-Sonic nebulizers. The choice of the most appropriate nebulizer depends on the pharmaceutical being administered, since aerosol must be delivered to the required site in sufficient quantity, whilst minimizing the incidence of local or systemic side-effects.