Abstract: An oscillating displacement pump particularly adapted for pumping a viscous and abrasive fluid medium or one loaded with solid particles. The pump includes a connecting space located outside of a displacement unit and directly connecting a suction valve space with a pressure valve space so that the geodetic height of pumped medium increases. The connecting space defines a suction valve side connecting channel and a pressure valve side connecting channel connected at any given time with each other through the displacement unit space independent of the position of the displacement unit. The pumped medium can thus be moved through the pump from the suction valve to the pressure valve without diversion in a separate connecting space reducing abrasive wear on the pump. A number of additional advantageous features of the pump are disclosed.