Abstract: A positive pressure positive displacement implantable infusion pump. A positive pressure fluid reservoir maintains infusate for delivery by a positive pressure displacement pump. Infusate is drawn by the pump into a drug pressurant chamber. A check valve prevents backflow into the reservoir. The check valve may independently establish a basal flow rate by not completely sealing in the forward flow direction. The pump pulses to expel and fill the chamber thereby establishing the flow rate, basal or bolus through a restrictor interposed between the chamber and the infusion site. The pump may run in open loop, pre-programmed or externally programmed modes of operation.