A metered dose inhaler for dispensing a fluid medicament as droplets of small mean particle size without the use of pressurized gas or liquefied propellants. A liquid drug is contained in a fluid reservoir, and metered quantities of the drug are successively presented into a reduced cross-section pressure chamber, with non-return valves controlling the flow of liquid through the device. The metered quantity of the drug is then subjected to a sudden and great increase in pressure by releasing a piston into the pressure chamber. The liquid drug is thereby ejected through an atomizing head, to reduce it to a fine atomized spray of small mean particle size, preferably less than approximately 12 micrometers.