An implantable microinfusion pump system for dispensing medication at a prescribed infusion rate within the human body includes a hermetically sealed housing in which a power source and circuitry for actuating the pump are contained. A permanently magnetized piston is mounted for reciprocative fill and pump strokes within a pump chamber defined by a pump body member which extends between parallel-spaced walls of the housing. The body member communicates at one end with an expandable medicament reservoir adjacent one wall of the housing, and at the other end with a catheter for conveying medicament to the body. A two stage valve arrangement causes medicament to enter the pump chamber during fill strokes of short duration and to be dispensed from the pump chamber at the prescribed infusion rate during longer duration pump strokes. To minimize current drain from the power source the piston is driven through the shorter duration fill strokes by magnetic actuator windings within the housing, and through the longer duration pump strokes by magnetic attraction between the piston and a magnetic pole piece.