In one aspect, the present invention is directed to an osmotic pump that automatically provides an ascending release rate of active agent as the osmotic pump functions in an environment of operation and may be designed for implantation within a desired animal or human subject. An osmotic pump according to the present invention includes a reservoir, a rate controlling membrane, an expandable osmotic composition, an active agent formulation and an exit orifice. Once administered to an environment of operation, water passes through the rate controlling membrane and into the osmotic composition, which causes the osmotic composition to expand and expel the active agent formulation through the exit orifice at a rate that is directly proportional to the rate at which water passes through the rate controlling membrane. An osmotic pump according to the present invention permits the flow of water through the rate controlling membrane to increase automatically without the need for manipulation of the osmotic pump after administration. As the flow of water through the rate controlling membrane increases, the rate at which active agent is delivered from the osmotic pump will also increase proportionally.