A pump primarily for liquid paint comprising first and second pistons (19, 21) reciprocable rectilinearly in respective first and second cylinders (17, 18), said first and second pistons being moved relative to their respective pistons by operation of an A. C. electric motor (13) the rotary output shaft of which is coupled to said first and second pistons by means including a constant velocity cam (31) and cam follower (32, 33) mechanism converting rotary motion of the output shaft into reciprocatory motion of said first and second pistons 180° out of phase with one another.