A system is disclosed for demand-based administration of insulin which utilizes a change in a physical property of a patient's body fluid as an indicator that a change in the body fluid glucose level has occurred and that an appropriate amount of insulin should be supplied to the patient. Body fluid osmolality or one of its colligative properties are particularly suitable for this purpose. A second sensor for measuring such properties as electrical conductivity may be added to detect changes in osmolality caused by such factors as dehydration. Insulin can be delivered to the patient from an extracorporeal source by means of a dual lumen catheter. One lumen of the catheter provides a passageway for the insulin dose and the other lumen provides a passageway through which a replaceable sensors can be introduced into or removed from the patient's body. In the alternative, the entire insulin administration system can be implanted into the patient.