Abstract: A mixing pump includes a primary pumping chamber opposed by a secondary mixing chamber with primary and secondary pistons that are connected to each other and reciprocated in the chambers. Preferably, an actuator mechanism that drives the pistons is located between the chambers. An additive fluid is expelled from the secondary chamber through a mixing conduit into a mixing chamber where it is injected into a main fluid. The fluid mixture from the mixing chamber is then admitted to the primary pumping chamber from which it is expelled under pressure, preferably through an outlet valve in the primary piston. The fluid can be more thoroughly mixed as it passes through that valve and an annular passage surrounding this piston.