Apparatus for repeatedly metering and dispensing, by means of a piston pump, predetermined quantities of liquid via a switching valve to which are connected inlet and outlet conduits for the liquid. The pump and the valve are actuated in timed relation by a drive mechanism powered by an electric motor which is controlled by selectively operable means that automatically cut off the motor supply once the piston has completed a stroke. There are also provided means for varying the length of the piston stroke to adjust the volume of said quantities and means for displaying to the outside a number indicative of this volume.