Abstract: An alarm system for a respirator fed from a supply of gas under pressure comprises a control valve having a pilot port and feed ports displaceable between an open position permitting fluid flow between the feed ports and a closed position preventing such flow. A pilot line connected to the pilot port is also operatively connected to the respirator for feeding gas under pressure from the respirator to the pilot port and urging the valve into one of the positions, the pressure in this pilot line being the pressure being applied to the patient. A spring is braced against the valve and urges same into the other position. A pneumatically actuatable acoustic alarm is connected to one of the feed port. Gas is fed substantially continuously under pressure to the other feed port. Thus when the pilot line pressure drops the valve moves into the other position and the alarm is either actuated or its actuation is stopped.