A piston proportioning pump for liquids, with a piston in a cylinder, an uptake and discharge valve and an oscillating drive is improved so that the formation of vapor bubbles or of a low pressure during the suction phase is prevented even when the pump space is filled quickly. The dead space of the piston proportioning pump still remains so small that a largely complete expulsion of the proportioned amount through the discharge valve is guaranteed. To this end, provisions are made so that the discharge communicates with the pump space within an uptake valve.