A power injector is provided for injecting fluid from a replaceable syringe and through an injection tube into the body of an animal. The syringe includes one or more operator-removable and interchangeable syringe mounting heads, at least one of which mounts syringes of a rear loadable type. Preferably an alternative head is provided that accepts front loadable syringes. The mounting head includes an opening and a moveable syringe holder that preferably pivots open to receive a syringe from the back of the holder and pivots closed to orient the rear of the syringe over the ram. The holder has a slot therethrough having a width sufficient to permit an injection tube attached to the syringe nozzle to be moved transversely through the slot to permit the syringe to be removed from the holder without disconnecting the tube from the orifice of the syringe and without withdrawing the tip of the tube rearwardly through the opening. The holder preferably includes a replaceable insert configured to snugly support a rear loadable syringe of a given configuration. One insert is preferably provided in the form of a pressure sleeve configured to support a syringe during high pressure use. The head has a locking mechanism that locks the syringe to the injector, couples the syringe plunger to the ram, locks and releases the head and locks the holder in operating position.