A procedure for measuring the effects of flow increase rate (FIR) on mouth-throat and downstream filter ("lung") deposition was developed. A respiratory aerosol probe (RAP) was used for generation and delivery of small aerosol boluses. This device employs computer controlled, fast opening and closing valves, which allow aerosol injection at a predetermined time during inhalation. Flow was driven by a pulmonary waveform generator (PWG) breathing machine. Monodisperse, 5 mu m diameter dioctylphthalate (DOP) particles were used. The following experimental flow conditions were tested: bolus injection into a steady-state flow of 30 1/min, and bolus injection into unsteady flow accelerating through 30 1/min at either 2 or 41/s2. In addition, numerical simulation of the effects of FIR on mouth-throat deposition of aerosol boluses was performed. Our results show that unsteady flow resulted in enhanced mouth-throat deposition. This difference can be explained by the higher velocity reached by the particles at their primary deposition site within the mouth-throat due to the accelerating flow rate (which reaches approximately 40 1/min by the time the particles deposit in the area of the larynx). This hypothesis is supported by data with bolus injection into a steady flow rate of 40 1/min, which yields mouth-throat deposition that does not differ significantly from the unsteady case. (c) 2006 Elsevier Ltd. All rights reserved.