Abstract: A diffusional drug delivery device is described which can provide for delayed onset of therapeutic effect and for delivery of the therapeutic agent in predetermined temporal patterns at optimum rates. Delay means are provided between the agent reservoir and the surface through which the agent is released by diffusion to delay the release of agent at a therapeutic rate for predetermined times after application. Suitable means include a delay membrane disposed between the releasing surface and the agent reservoir which membrane is preferably free of undissolved agent and/or is initially impermeable to the agent and thereafter becomes permeable. One or more agent chambers defined by one or more of such membranes may be provided, whereby agents are released in a predetermined temporal pattern at optimal release rates.