A medical device comprising a fluid imbibing, self-regulating, skin patch drug delivery system is disclosed which includes a fluid imbibing, preferably osmotic, pump comprising a reservoir for a drug or, drug formulation or other fluid to be dispensed to the skin, 10 at least one wall of the reservoir comprising a fluid permeable membrane which is permeable to an external activating fluid. The external fluid is imbibed through the membrane at a predetermined rate thereby causing the drug to be pumped through one or more passages provided in the reservoir structure and 15 into contact with the skin. The device includes an impermeable exterior wall member which covers and extends outwardly from the fluid imbibing pump terminating in an edge portion adapted to be secured, as by adhesion, to the skin. The exterior wall member, when attached to the skin surface, defines the area of skin which is available for distribution of the drug formulation pumped from the reservoir which is large enough, at the lowest anticipated skin permeation, to deliver the drug to the body at the rate at which it is discharged from the fluid imbibing pump in the preferred embodiments the fluid permeable membrane is a semipermeable membrane, impermeable to the drug or drug formulation.