PIP: Controlled delivery systems using rate-limiting membranes are described, and mention is made of their potential application in birth control devices. In general, such systems 1) remove the necessity of repeated redosing, 2) allow smaller amounts of the active agent to produce the desired effect, and 3) deliver the agent locally and contain it at the site of action. Work is now progressing on erodible delivery systems which would allow the device to self-destruct after use. This could be applied to IUD in underdeveloped countries where return of the patient for device removal cannot be assured.