The application of positive end-expiratory pressure (PEEP) as part of artificial ventilation is currently considered a basic provision to provide adequate treatment of critically ill patients in intensive care units. Hitherto only the application of intermittent positive pressure ventilation (IPPV) with an end-expiratory pressure of 0 cm water was possible during manually operated artificial ventilation. A new PEEP valve which can be attached to manually operated resuscitators has been developed in co-operation with industry to meet this need. The technical details and experimental testing of the new valve have been presented in this paper. It may be connected via special adaptors to existing Ambu resuscitators, either for use on adults or on neonates, babies and infants. The valve has the characteristics of normal PEEP devices in that the resistances with various PEEP levels remain relatively independent of the flow and thus presents a true positive end-expiratory pressure rather than just an increase of expiratory resistance. The valve can be connected simply to nearly all types of simple and advanced respirators, such as those used for assisting respiration, physiotherapy etc., as well as being used with manually operated resuscitators.