Abstract: An apparatus and method for humidifying gases, suitable for use in conjunction with a high frequency positive pressure ventilation (HFPPV) respiratory system, includes a nebulizing chamber 11 arranged to receive a pulsed gas flow from an oscillating valve 5 and a liquid reservoir 15 communicating with the chamber 11 such that in use liquid from the reservoir 15 passes into the chamber and is entrained in droplet form by the gas flow. A pump 20 is intermittently operable to introduce a predetermined quantity of liquid into the reservoir 15, and control means 6 of the apparatus are adapted to maintain the valve 5 in its closed condition during operation of the pump 20.