Abstract: The invention relates to a device suitable for use in the intermittent or even continuous administration of a therapeutic substance such as insulin or morphine.The device comprises a cannula of biocompatible plastics material for indwelling in a patient and defining a through-passsage, together with a hub also defining a through-passage. The respective cannula and hub passages are isolatable from each other by separator means such as a self-sealing diaphragm disposed between them. In the device the arrangement of the respective passages themselves together with said diaphragm or the like is such as to permit a needle to penetrate the hub and cannula through them and, when necessary, to extend beyond the distal end of the cannula. The through-passage of the hub is also constructed and arranged so that it guides an injection needle passed therethrough to pass through the diaphragm at a position where the needle is aligned with the through-passage of the cannula.