An improved infusion pump is provided for controlled delivery of medication from a syringe to a patient, wherein a pump housing includes a syringe chamber and associated lock members for receiving and supporting the syringe in one of two different positions. The syringe includes a medication-containing barrel connected to a luer fitting at a nose end thereof by a luer neck of reduced cross sectional size. In one position, the syringe seats into the chamber with the luer neck protruding through an outlet port of mating size and with the luer fitting disposed outside the housing, such that the housing defines a lock member engaged axially between the syringe barrel and luer fitting to secure the barrel against axial displacement. In another position, a retainer key is adapted for installation into the syringe chamber at a position spaced inboard from the outlet port and defines an alternative lock member for axially securing the syringe with the luer fitting disposed inside the housing. In either position, a syringe piston plunger is controllably advanced by the pump into the barrel to deliver medication from the nose end of the syringe for administration to a patient via the luer fitting and infusion tubing or the like connected thereto.