An osmotic delivery system for controlled delivery of a beneficial agent includes an implantable capsule containing a beneficial agent and an osmotic engine that swells on contact with water, thereby causing the release of the beneficial agent over time. The osmotic delivery system has a membrane material that allows a controlled amount of fluid to enter from an exterior of the capsule, while preventing the compositions within the capsule from passing out of the capsule. The osmotic delivery system is designed to meet at least the operating pressures of 1000 psi. The membrane material is cast, calendered or extruded followed by machining (i.e., die-cutting, stamping or otherwise cutting to shape) to provide a uniform nonribbed membrane material. The capsule also includes a membrane material-retaining means that is positioned at a fluid uptake end to retain the membrane material within the capsule, even under periods of high pressure.