A reciprocating pump mechanism having a pump body forming a fluid inlet chamber and opposed pump cavities in communication with the fluid inlet chamber. The pump employs a piston having opposed pumping extremities adapted for pumping entry into the respective pumping cavities and is of a length such that with one pumping extremity positioned to its full extent within a pumping cavity the opposite pumping extremity there of is disposed in fluid inletting relation with the opposite pumping cavity. The piston may also include opposed drive shafts at least one of which extends from the pump body. One of the piston shafts may be adapted to be driven by any suitable reciprocating prime mover while the opposite piston shaft is adapted for connection to a similar exposed piston drive shaft of a similar double-acting piston pump to thereby provide for connection of the pumps in tandem driven relation by a single prime mover. The pump mechanism also incorporates a single pump discharge having a pair of check valves in unidirectional fluid controlling communication with the respective piston cavities.