Abstract: A rotary/reciprocating liquid dispensing pump for dispensing liquids in the nanoliter range has a narrow radial slot extending longitudinally from an end of a reciprocating, rotating cylindrical piston sliding in a smooth cylindrical bore of a pump housing at a closed end thereof, which slot extends outwardly to the periphery of the piston. The piston is selectively positioned in alignment with one of a plurality of circumferentially spaced small diameter radial passages within the housing opening directly to the cylindrical bore in the path of rotation of said slot. The ports thereof opening to the cylindrical bore are spaced circumferentially a distance in excess of the width of the slot. A linear liquid dispensing pump apparatus including such pump includes a stepping motor and an electromagnetic clutch/braking mechanism concentrically surrounding a lead screw, and the lead screw is threadably engaged with a stepping motor tubular rotor shaft.