An osmotic system for delivering a beneficial agent is disclosed. The system comprises a wall surrounding a compartment and has a passageway through the wall for delivering agent from the compartment. The wall is formed of a material permeable to the passage of an external fluid and impermeable to the passage of agent. The compartment contains (1) an agent that is soluble in the fluid and exhibits an osmotic pressure gradient across the wall against the fluid, or (2) the compartment contains an agent having limited solubility in the fluid and exhibits a limited osmotic pressure gradient across the wall against the fluid. The compartment also contains along with (1) or (2) a volume amplifier for increasing the amount of agent delivered from the system. The amplifier comprises a membrane surrounding a gas generating couple with the membrane formed of an expandable material permeable to fluid and impermeable to the couple. In operation, agent is delivered from the system through the passageway at a controlled rate by fluid being imbibed through the wall into the compartment to produce a solution of (1), or a suspension of (2), and for imbibition by the amplifier causing the couple to generate gas and continuously fill the amplifier thereby urging it to increase in volume and fill the compartment whereby agent (1) or (2) is released at a rate controlled by the permeability of the wall, the osmotic pressure gradient across the wall, and the rate of imbibition and increase in volume of the amplifier over a prolonged period of time.