Abstract: A method and apparatus for providing a disinfectant cycle of a medical device such as a dental device having an input water line with a main valve and a plurality of water lines extending to instruments with control valves characterized by an arrangement for injecting a disinfectant into the input line downstream of the main valve and an arrangement for controlling the opening and closing of the various valves to enable replacing the water in the water lines with disinfectant, and subsequently flushing the disinfectant therefrom. The method and apparatus can include a device for dosing the water flowing through the water lines during usage with a disinfectant to retard growth of microorganisms and the apparatus can include a selector switch to select various modes of operation.