A totally implantable infusion pump has been developed that can provide chronic, continuous, low-rate drug infusions in ambulatory patients. The device relies on vapor pressure for its power source, and thus needs no recharging other than periodic drug refills. It is implanted surgically under the skin of the chest or abdomen, with the delivery cannula placed in an appropriate blood vessel. Pumps implanted in this way have delivered heparin, 5-fluorodeoxyuridine, or other drugs in patients; some outpatients have received continuous intravenous heparin infusions for more than 4 years.