Therapeutic aerosols pay an increasing role in the treatment of equine respiratory disorders. This route of delivery permits concentration of significant amounts of drugs at the site of action without unwanted high systemic concentration and resultant side effects. The efficiency of such a topical therapy depends on the quantity of inhaled drugs deposited in the lungs and, for some drugs, on the proportion retained in specific parts of the lungs. The objective of this study was to define and to compare quantitative (dose deposited) and qualitative (regional distribution) deposition of an aerosol in the equine lungs, using either a ultrasonic nebuliser (UN) currently used in human medicine or a high pressure jet nebuliser (JN) especially developed for the equine species. This comparison was possible owing to gamma-scintigraphy, a noninvasive technique ideally suited to give information about both total and regional deposition of inhaled drugs in the respiratory tract. The quantitative study did not point out any difference between the 2 systems concerning the activity released from the nebuliser proportionally to the initial loaded dose (mean +/- s.d. 45.95 +/- 4.93% for the UN vs. 46.47 +/- 8.49% for the JN). By contrast, the percentage of the dose released reaching the lungs was significantly lower with the UN compared to the JN (5.09 +/- 0.66% vs. 7.35 +/- 1.96%). The qualitative analysis did not show any significant difference in size of aerosol deposition image between the 2 nebulisers. However peripheral deposition was significantly higher with JN compared to UN. In conclusion, both nebulisers may be used for aerosol therapy in the equine species. The ultrasonic and pneumatic nebulisation achieved drug deposition in the peripheral part of the lungs (i.e. small airways and lung parenchyma).