An infusion system is disclosed for delivering precisely regulated and variable dosages of drugs. The device includes a reservoir for containing the drug, a catheter for delivering drug to the body, and actuating means responsive to a signal applied externally of the body for initiating delivery of a precisely regulated dosage. The actuating means include; a solenoid driven miniature pump which is controlled by an implanted controller and driven by an implanted power source. The implanted controller provides a basal dosage rate which itself may be variable or constant and which may be altered by telemetry signals delivered from outside the body. In addition, the internal device is operable by an external unit which may be set at a different rate from that which the internal or implanted controller is set and which, in addition, provides power for operating the implanted device. The reservoir is maintained at zero gauge or slightly negative gauge pressure to prevent loss of the contents of the reservoir into the body in which the device is implanted in the event of a failure. A tube communicating between the reservoir and the catheter is bent around on itself in such a way as to prevent bubbles from entering the tube. The implanted power and control electronics may be overridden to provide power from an extra corporeal unit and is also programmable from the extra corporeal unit to adjust dosage rates. A bellows is used to maintain a constant pressure within the reservoir, and suitable plates are utilized to produce a capacitive signal indicating the volume remaining of the contents of the reservoir.