The construction and operation of a compact automatic Toepler pump of the mercury piston type which has many advantages over previous designs are described. The mercury motion is controlled by a switching relay activated by electrodes so placed that at the ultimate of its stroke the air leak to the lower mercury reservoir is closed and an oil vacuum pump is started to evacuate it, and at the bottom of the stroke the vacuum pump is stopped and the air leak opened to allow the mercury to rise in the piston chamber. Pumping action is effected by two glass valves with conically ground seats in series in the gas inlet and outlet tubes.