A syringe includes a threaded plunger disposed for axial movement within a housing. When it is designed to aspirate a fluid into the syringe barrel, a cam member mounted on the barrel is moved to a free flow position in which it is disengaged from the plunger to allow the plunger to be freely axially movable along the barrel. When it is desired to inject the fluid, such as into an angioplasty balloon, the locking cam is moved to an engaged or locking position, in which it threadably engages the plunger, such that the plunger can only be moved along the barrel in small, controlled increments by rotating the plunger, thereby to enable the operator to achieve the desired accurate control over the fluid pressure in the syringe.