1. In apparatus for intermittently supplying respiratory gas to a spontaneously breathing subject which is supplemental to said subject's normal ambient air intake including a gas supply connection means adapted for connection to a gas supply and a subject connection means adapted for delivering said gas to said subject without substantially obstructing said subject's access to ambient air, the improvements comprising: gas conduit means adapted for connection between said gas supply connection means and said subject connection means to provide a fluid path for said gas; valve means interposed in said gas conduit means for controlling an intermittent flow of said gas by sequentially and repetitively opening said valve means for a predetermined number of ON respiratory cycles and closing said valve means for a predetermined number of OFF respiratory cycles; respiratory cycle sensing means adapted for connection to said subject for sensing the respiratory cycles of said subject; counter means connected to said sensing means for summing the number of respiratory cycles; counter presetting means connected to said counter means, causing said counter means to respond when a preset number of cycles has been registered on said counter means, said presetting means including means for storing at least one ON number representing a predetermined number of ON respiratory cycles and at least one OFF number representing a predetermined number of OFF respiratory cycles; valve controller means connected between said counter means and said valve means, wherein said valve controller means is responsive to the response of said counter means upon registration of said preset numbers by operating said valve means, wherein said valve controller means turns said valve means on when a preset OFF number has been registered by said counter means and turns said valve means off when a preset ON number has been registered by said counter means to conserve said gas supply by providing a flow of supplemental gas only during those respiratory cycles when it can be most useful and conserving said gas during other respiratory cycles when it will be less useful.