An improved reservoir is provided for use in a medication infusion device for operating the reservoir to administer a fluid to a patient. The reservoir comprises a hollow barrel adapted to receive a supply of fluid for delivery through infusion tubing or the like to the patient, in combination with a reservoir plunger. The reservoir plunger is designed to engage a motor-driven lead screw of the medication infusion device. The reservoir plunger has an integral split nut connector formed on the end of the plunger, which is in co-axial alignment with the lead screw whereby the split nut connector can be engaged directly to the lead screw.