An apparatus for downhole injection and mixing of fluids into a cement slurry. The apparatus includes embodiments which either carry a fluid to the proper injection point in a wellbore or maintain it at the proper injection point until activated. In a first embodiment, the apparatus is characterized as a cementing plug having a fluid holding chamber therein. When the plug is pumped downwardly in the wellbore, the fluid in the chamber is flowed out of the chamber and mixed with the cement slurry by a venturi effect. In a second embodiment, the apparatus is characterized by a casing portion having a fluid holding chamber therein. A solenoid valve controls communication of the chamber with the well annulus, and fluid flow through the well annulus causes the fluid in the chamber to be flowed outwardly by a venturi effect. The second embodiment includes a casing shoe disposed below the fluid holding chamber. A third embodiment is disclosed which is substantially the same as the second embodiment except that it utilizes a casing cementing valve rather than a casing shoe. Methods of utilizing the apparatus to cement a well casing and mix a fluid, such as an accelerant, with the cement slurry are also disclosed.