A pump primarily for liquid paint includes first and second pistons reciprocable rectilinearly in respective first and second cylinders. The first and second pistons are moved relative to their respective pistons by operation of an A. C. electric motor the rotary output shaft of which is coupled to the first and second pistons by a constant velocity cam and a cam follower mechanism converting rotary motion of the output shaft into reciprocatory motion of the first and second pistons 180° out of phase with one another.