A portable grease gun with a dual pressure output capability which can be selectively changed by the user with little effort. The larger diameter plunger is slidable within a plunger retainer. The larger diameter plunger has a bore through them in which a smaller diameter plunger is slidable. The smaller diameter plunger moves when an actuating lever is operated to compress grease in the grease gun. The larger diameter plunger may be adjusted to remain static during operation of the actuating lever or it may be adjusted to move during all or part of the compression stroke of the actuating lever. A high pressure can be generated by movement of the smaller diameter plunger alone, or a high flow volume can be generated by movement of the smaller and larger diameter plungers in unison.