Abstract: A medication infusion system includes an implantable device and an external control device. The implantable device includes a constant flow pump and a flow rate that may be altered by commands received from the external control device. Power to execute the commands is also provided to the passive pump from the external control device. The external control device sends a command only when a flow rate change is to be made, or when status information is required. In one embodiment, the flow rate provided by the passive pump is controlled by a flow regulator, where the flow rate is changed by changing the restriction of a passage between regulator chambers.