A fluid end with an internal bellows can be used in high pressure environments. The bellows is disposed within a plunger bore, and has an interior isolated from a working chamber of the fluid end. An end of a plunger is received in the interior of the bellows. The bellows contains an incompressible fluid at a constant volume. As the plunger extends into the interior of the bellows, the fluid it contains is displaced. Such fluid displacement causes the bellows to extend. As the plunger is retracted, the fluid replaces the volume formerly displaced by the plunger, and causes the bellows to accordion to a retracted position.