Abstract: A nebulizer for use in connection with a lung ventilating system characterized in providing a dual stage atomization of medicament carrying liquid. A first stage develops an aerosol exhibiting an initial, relatively larger average liquid particle size which is introduced to a chamber at a given stagnation pressure. From that chamber, a second atomization is carried out by passing the gas entrained liquid particles through a knife edge orifice to produce a rapid expansion free-jet flow and consequent particle breakdown to achieve a desired particle size distribution.