An implantable device for releasing one or more substances into a body cavity such as a urinary bladder, is provided. The device includes an inner balloon located within an outer balloon so as to define a lumen between the outer and inner balloons. A first valve is used for filling the lumen; and a second valve is used for filling the inner balloon. The device is delivered to the body using a delivery catheter. The inner portion is filled with the substance or substances to be released into the body cavity. The outer balloon is filled with a substance that is chosen to impart a desired specific gravity to the device. The substances to be released may be contained in particles coated with a soluble coating. The particles may be in suspension or embedded in a matrix.