A ventilator which includes: at least one pair of reciprocating opposed gas moving means, the internal volume of which is arranged to be reduced and enlarged by a balanced drive, said gas moving means being arranged to move gas at a predetermined pressure and/or volume to a delivery tube control means for regulating the speed and the distance of movement of the drive wherein the masses of all moving parts are balanced so as to minimise the vibration of the ventilator in use.
