A selectively permeable membrane (14) is positioned between the agent reservoir (15) and the electrode (11) of a donor electrode assembly (8) in an iontophoretic delivery device (10). Optionally, an electrolyte reservoir (13) is positioned intermediate the electrode (11) and the agent reservoir (15). In certain embodiments, the membrane (14) is permeable to species of less than a predetermined molecular weight and substantially less permeable to species of greater than the predetermined molecular weight. The agent is capable of dissociating into agent ions and counter ions. The size selectivity of the membrane (14), the molecular weight of the agent ions, the molecular weight of the counter ions, and optionally the molecular weight of the electrolyte ions are selected to (A) inhibit agent ions from interacting with the electrode (11), and from permeating into the electrolyte reservoir (13), where they might otherwise be degraded and (B) inhibit electrolyte ions, having a charge similar to that of the agent ions, from permeating into the agent reservoir (15) and competing with the agent ions for iontophoretic delivery into the patient.