Abstract: A liquid metering injector assembly for injecting metered quantities of a first liquid into respective metered quantities of a second liquid comprising a metering cylinder, metering ports formed at either end of the metering cylinder; a metering piston displaceable in the metering cylinder; a valve unit having a valve inlet for coupling to a pressurized source of the first liquid; means for coupling the valve to a hydraulic pulse source; a valve outlet for coupling to a source of the second liquid; a pair of communication valve ports of said valve unit respectively communicating with the metering ports and selectively communicatable with the valve inlet and outlet and a displaceable spool valve of the valve unit responsive to the hydraulic pulses so as to be displaceable between two positions in which the respective communication of the valve unit communicating ports with the valve unit inlet and outlet are successively reversed.