Abstract: This invention relates to the monitoring of the correct operation of a respirator with single source of energy constituted by a cylinder of pressurized oxygen, to which is annexed a device for connection to a patient. Such monitoring is effected with the aid of a pneumatically operating device which comprises a comparator with adjustable threshold, connected to the mask for connecting to the patient and followed by an amplifier whose output signal takes the binary logic level 1 during the time when the insufflation pressure in the mask is greater than said threshold, and takes the other binary logic level 0 in the opposite case, and a circuit for processing this signal, which triggers off an alarm device if the duration of the time intervals when this latter is at level 1 is too long or too short with respect to a predetermined normal value.