Abstract: Inflating and deflating device for a balloon dilatation catheter comprising a housing and a syringe carried by the housing. The syringe has an outlet adapted to be connected to the catheter. The syringe includes a container and a piston which is slidably mounted in the container. The syringe also includes a rigid plunger connected to the piston and extends out of the housing. The plunger has helical threads formed thereon to provide an elongate ratchet. A pawl is movable between ratchet-engaging and ratchet-disengaging positions. A spring is carried by the housing and engages the pawl and yieldably urges the pawl into its ratchet-engaging position. A tab is carried by the housing and engages the pawl and is used for moving the pawl out of engagement with the ratchet to release the ratchet.