An implantable infusion apparatus, comprising: a housing defining a pressure chamber that contains a fluid having positive vapor pressure at normal body temperature; at least a first infusate reservoir and a second infusate reservoir respectively defined by bellows (33, 34) in the pressure chamber; a metering assembly comprising an accumulator having an inlet and an outlet; a first valve in fluid communication with the first infusate reservoir and the accumulator; a second valve in fluid communication with the second infusate reservoir and the accumulator; and an outlet valve in fluid communication with the outlet of the accumulator; the metering assembly having a fixed volume, the accumulator comprising a chamber housing a flexible diaphragm that provides a barrier between a gas portion of the chamber and an infusate portion of the chamber; wherein the first infusate reservoir and the second infusate reservoir are maintained at a positive pressure by the fluid that is external to the first infusate reservoir and the second infusate reservoir; wherein the second infusate reservoir is oppositely disposed to the first infusate reservoir; wherein first infusate reservoir and second infusate reservoir are individually expandable to a selected portion respectively of a total available volume of the pressure chamber, the first infusate reservoir expanding in a first direction and the second infusate reservoir expanding in a second direction that is opposite the first direction; wherein when the first valve is open and the second valve and the outlet valve are closed, infusate from the first infusate reservoir is introduced into the accumulator; wherein when the outlet valve is open and the first valve and second valve are closed, infusate is expelled from the accumulator; wherein when the second valve is open and the first valve and the outlet valve are closed, infusate from the second infusate reservoir is introduced into the accumulator; and wherein when the outlet valve is open and the first valve and second valve are closed, infusate is expelled from the accumulator. a housing defining a pressure chamber that contains a fluid having positive vapor pressure at normal body temperature; at least a first infusate reservoir and a second infusate reservoir respectively defined by bellows (33, 34) in the pressure chamber; a metering assembly comprising an accumulator having an inlet and an outlet; a first valve in fluid communication with the first infusate reservoir and the accumulator; a second valve in fluid communication with the second infusate reservoir and the accumulator; and an outlet valve in fluid communication with the outlet of the accumulator; the metering assembly having a fixed volume, the accumulator comprising a chamber housing a flexible diaphragm that provides a barrier between a gas portion of the chamber and an infusate portion of the chamber; wherein the first infusate reservoir and the second infusate reservoir are maintained at a positive pressure by the fluid that is external to the first infusate reservoir and the second infusate reservoir; wherein the second infusate reservoir is oppositely disposed to the first infusate reservoir; wherein first infusate reservoir and second infusate reservoir are individually expandable to a selected portion respectively of a total available volume of the pressure chamber, the first infusate reservoir expanding in a first direction and the second infusate reservoir expanding in a second direction that is opposite the first direction; wherein when the first valve is open and the second valve and the outlet valve are closed, infusate from the first infusate reservoir is introduced into the accumulator; wherein when the outlet valve is open and the first valve and second valve are closed, infusate is expelled from the accumulator; wherein when the second valve is open and the first valve and the outlet valve are closed, infusate from the second infusate reservoir is introduced into the accumulator; and wherein when the outlet valve is open and the first valve and second valve are closed, infusate is expelled from the accumulator.