A device for feeding a treatment liquid into medical appliances. The device includes an outlet opening for the treatment liquid, especially for feeding water into dental equipment, a feed line for the treatment liquid, means to introduce degerminating agents into the treatment liquid, and an on/off valve disposed upstream from an outlet opening onto which the feed line is connected. A back-flush valve is connected to the on/off valve, and a reservoir is provided for the treatment liquid, whereby the treatment liquid is transported from the reservoir with the aid of a pump. Further, a back-flush line is connectible alternatively via a drain valve to a drain or to the reservoir so that the treatment liquid is pumped into the drain, or in circulation from the reservoir through the feed line to the switch valve and through the back-flush line again into the reservoir.