A metering pump that is capable of delivering a constant, rate of fluid over a wide range of viscosities at infinitely variable delivery rates. Two identical pumping chambers are provided with one operating in the suction mode, and the other in the discharge mode with each cylinder switching modes at each pumping cycle, thereby providing continuous delivery of fluid in and out of the pump. The displacement of the chamber is large in order to facilitate operation at a very slow cycle speed of only one to four reciprocations per minute. The fluid valves are mechanically operated providing for positive sealing thus enabling the pump to operate at very slow speeds without leakage.