Abstract: An ambulatory parenteral fluid infusion pump employs a disposable in-line cassette which provides three independent fluid paths between two flexible plastic sheets. The fluid path extends through a pump chamber having a piston plate secured to the flexible sheet at each pumping chamber. The sealed flexible sheets are housed in a rigid housing which provides an aperture through which a catch member formed on the piston plate extends, and having living hinges overlying the inlet and outlet paths for shutting off those paths. An outlet valve is normally closed, and operates in response to buildup of fluid pressure from advancement of the piston plate into the pump chamber to deliver fluid.