To determine the effect of aerosol particle size upon absorption from the respiratory tract (RT), solid, polydisperse disodium fluorescein aerosols (MMDae 1.1, 3.5 and 4.4 micron) were delivered under the same respiratory regime direct to the RT of 2 Beagle dogs by positive pressure ventilation using a purpose designed administration system. Subsequent to aerosol administration the amount of fluorescein absorbed as a function of time was estimated from plasma concentration versus time and intravenous control data using a modified Loo Riegelman method. Maximum fractional deposition within the RT occurred with 3.5 micron aerosol. Fluorescein was absorbed rapidly from the RT according to an apparent first-order process the rate constant for which was independent of particle size and possibly regional deposition. Average values for absorption half lives in the 2 dogs were 17.3 and 11.4 minutes.